Crossword Genius

Knock-knock joke, basically

Ross

I believe the answer is:

pun

(Other definitions for pun that I've seen before include "Double entendre" , "Humorous wordplay" , "A play on words, not really wit" , "Humorous play on words" , "A play on words - not the height of wit" .)

I've seen this clue in The Mirror.
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