Play on words (3)
I believe the answer is:
'play on words' is the definition.
This is the entire clue.
(Other definitions for pun that I've seen before include "Bit of paronomasia; witticism" , "Form of wordplay" , "A play on words, perhaps the lowest form of wit" , "A play on words - not the height of wit" , "A play on words, not really wit" .)