Are you saying that it is OK for a preacher to tell jokes behind the pulpit if they are of a clean sort?

Answer:

"But fornication and all uncleanness or covetousness, let it not even be named among you, as is fitting for saints; neither filthiness, nor foolish talking, nor coarse jesting, which are not fitting, but rather giving of thanks. For this you know, that no fornicator, unclean person, nor covetous man, who is an idolater, has any inheritance in the kingdom of Christ and God" (Ephesians 5:3-5).

Paul is not talking about all jokes. He is speaking dirty talk and jokes about sexual things. Things that are now commonly referred to as "bathroom humor." See: What are the definitions of "foolish talk" and "coarse jesting?"

Humor is not wrong. The Bible contains many examples:

Sarcasm

• "Then Achish said to his servants, "Look, you see the man is insane. Why have you brought him to me? Have I need of madmen, that you have brought this fellow to play the madman in my presence? Shall this fellow come into my house?" " (I Samuel 21:14-15).
• "Yet you have forsaken Me and served other gods. Therefore I will deliver you no more. Go and cry out to the gods which you have chosen; let them deliver you in your time of distress" (Judges 10:13-14).
• "Their idols are silver and gold, the work of men's hands.
  They have mouths, but they do not speak;
  Eyes they have, but they do not see;
  They have ears, but they do not hear;
  Noses they have, but they do not smell;
  They have hands, but they do not handle;
  Feet they have, but they do not walk;
  Nor do they mutter through their throat.
  Those who make them are like them;
  So is everyone who trusts in them" (Psalms 115:4-8).
• "And so it was, at noon, that Elijah mocked them and said, "Cry aloud, for he is a god; either he is meditating, or he is busy, or he is on a journey, or perhaps he is sleeping and must be awakened" " (I Kings 18:27).

Irony

• "A classic example of irony in the Hebrew Bible is in the story of Joseph. Judah and his brothers had perpetuated one of the most horrible crimes one can imagine: They sold their own seventeen-year-old half-brother into slavery. Twenty-two years later the brothers had all but forgotten the evil they had done, but God did not forget. There was a famine in the Land of Canaan and the brothers were forced to go to Egypt to purchase food. Joseph, unbeknownst to his brothers, had become the Grand Vizier. Joseph’s silver chalice was found among Benjamin’s possessions and Joseph declared that Benjamin would have to remain in Egypt as his slave. Judah, in an attempt to get sympathy, told the Grand Vizier (in actuality Joseph) that they had an old father, and that Benjamin, the youngest child, was the only one of his mother’s children who was alive since he had had a brother who died. It is quite comical and ironic when one realizes that Judah was actually talking to Joseph who was far from dead. There is even more irony in the words of Judah whose plea before the Grand Vizier was (Genesis 44:34): "For how can I go up to my father if the lad [Benjamin] is not with me? I cannot bear to look upon the evil misery that shall come on my father." Twenty-two years earlier, Judah had no problem collaborating with his brothers in selling his seventeen-year-old brother and looking upon his father’s suffering. Indeed, Judah was the one who said (Genesis 37:26): "What profit will there be if we kill our brother…" It is also quite ironic that whereas at first Judah was the one who advised that one brother (Joseph) be sold into slavery, twenty-two years later, Judah offered himself as a slave in lieu of another brother (Benjamin). Judah redeemed himself by this noble act and thus we learn two important lessons: First, bad deeds can boomerang and cause one much harm in the future and, second, one can repent and be forgiven for even the worst of crimes" [Hershey H. Friedman, "Humor in the Bible"].
• "And Moses sent to call Dathan and Abiram the sons of Eliab, but they said, "We will not come up!" ... "So they and all those with them went down alive into the pit; the earth closed over them, and they perished from among the assembly" (Numbers 16:12, 33).
• "That night the king could not sleep. So one was commanded to bring the book of the records of the chronicles; and they were read before the king. And it was found written that Mordecai had told of Bigthana and Teresh, two of the king's eunuchs, the doorkeepers who had sought to lay hands on King Ahasuerus. Then the king said, "What honor or dignity has been bestowed on Mordecai for this?" And the king's servants who attended him said, "Nothing has been done for him." So the king said, "Who is in the court?" Now Haman had just entered the outer court of the king's palace to suggest that the king hang Mordecai on the gallows that he had prepared for him. The king's servants said to him, "Haman is there, standing in the court." And the king said, "Let him come in." So Haman came in, and the king asked him, "What shall be done for the man whom the king delights to honor?" Now Haman thought in his heart, "Whom would the king delight to honor more than me?" And Haman answered the king, "For the man whom the king delights to honor, "let a royal robe be brought which the king has worn, and a horse on which the king has ridden, which has a royal crest placed on its head. "Then let this robe and horse be delivered to the hand of one of the king's most noble princes, that he may array the man whom the king delights to honor. Then parade him on horseback through the city square, and proclaim before him: 'Thus shall it be done to the man whom the king delights to honor!'" Then the king said to Haman, "Hurry, take the robe and the horse, as you have suggested, and do so for Mordecai the Jew who sits within the king's gate! Leave nothing undone of all that you have spoken." So Haman took the robe and the horse, arrayed Mordecai and led him on horseback through the city square, and proclaimed before him, "Thus shall it be done to the man whom the king delights to honor!" (Esther 6:1-11). Haman is later hanged on the very gallows he had built to kill Mordecai.

Play on Words

• "So He humbled you, allowed you to hunger, and fed you with manna which you did not know nor did your fathers know, that He might make you know that man shall not live by bread alone; but man lives by every word that proceeds from the mouth of the LORD" (Deuteronomy 8:3). Normally food goes into a mouth, but man is to live by the spiritual food that comes out of God's mouth.
• "And Ehud came to him (now he was sitting upstairs in his cool private chamber). Then Ehud said, "I have a message from God for you." So he arose from his seat. Then Ehud reached with his left hand, took the dagger from his right thigh, and thrust it into his belly" (Judges 3:20-21). The Hebrew word for "message" also means "thing."
• "Then Samson said: "With the jawbone of a donkey, Heaps upon heaps, With the jawbone of a donkey I have slain a thousand men!" " (Judges 15:16). The Hebrew word for donkey and heap is the same.
• "And even as they did not like to retain God in their knowledge, God gave them over to a debased mind, to do those things which are not fitting" (Romans 1:28). “Like” translates the Greek word dokimazo, which means “approve of.” “Debase” or “reprobate” translates the opposite word, adokimos, which means rejected or disapproved of. Thus because they did not approve of having their minds on God, God gave them over to a mind disapproved of by God.
• "For our light affliction, which is but for a moment, is working for us a far more exceeding and eternal weight of glory, while we do not look at the things which are seen, but at the things which are not seen. For the things which are seen are temporary, but the things which are not seen are eternal" (II Corinthians 4:17-18).
• "So we are always confident, knowing that while we are at home in the body we are absent from the Lord. For we walk by faith, not by sight. We are confident, yes, well pleased rather to be absent from the body and to be present with the Lord. Therefore we make it our aim, whether present or absent, to be well pleasing to Him" (II Corinthians 5:6-9). The word endemeo, translated as “home” or "present," means to be in one’s own country or with one’s own people. “Absent” is translating the Greek word ekdemeo, which means to be away from one’s own country or people.

Imagery

• "Now they sin more and more, and have made themselves molten images of their silver, even idols according to their own understanding, all of them the work of the craftsmen. They say of them, 'They offer human sacrifice and kiss the calves.'" (Hosea 13:2 WEB). I quoted from the WEB translation because most translations missed the humor. Normally calves are sacrificed and people are kissed, but Hosea's point is that idolators get it backward.
• "As a door turns on its hinges, so does the lazy man on his bed" (Proverbs 26:14).

"A merry heart makes a cheerful countenance, but by sorrow of the heart the spirit is broken" (Proverbs 15:13).

"The light of the eyes rejoices the heart, and a good report makes the bones healthy" (Proverbs 15:30).