Judge copy to be a joke (4)
I believe the answer is:
jape
'be a joke' is the definition.
(I've seen this before)
'judge copy' is the wordplay.
'judge' becomes 'j'.
'copy' becomes 'ape' ('ape' can be a synonym of 'copy').
'j'+'ape'='JAPE'
'to' acts as a link.
(Other definitions for jape that I've seen before include "Practical joke, jest" , "Jolly wheeze" , "Trick - joke - jest" , "Trick or prank" , "Jolly joke" .)