Jack to copy joke (4)
I believe the answer is:
jape
'joke' is the definition.
(I know that joke can be written as jape)
'jack to copy' is the wordplay.
'jack' becomes 'J' (playing card abbreviation).
'to copy' becomes 'ape' (ape can mean to copy or imitate).
'j'+'ape'='JAPE'
(Other definitions for jape that I've seen before include "Lark" , "Kid" , "Trick - joke - jest" , "Jolly joke" , "Jolly wheeze" .)