Possible Clues:

• Cause for a groan
• Bad humor?
• A play on words, may be funny
• Many an 'Airplane!' gag
• 20-, 39- or 53-Across, say
• Many a crossword clue
• Bit of Cerf humor
• ''Mir—, Mir—, on the wall,'' e.g.
• That would be a funny sort of gent - breath-taking
• Semantic antic