Gag journalist's initial copy (4)
I believe the answer is:
jape
'gag' is the definition.
('jape' can be a synonym of 'gag')
'journalist's initial copy' is the wordplay.
'initial' suggests taking the first letters.
'copy' becomes 'ape' ('ape' can be a synonym of 'copy').
The initial letter of 'journalists' is 'j'.
'j'+'ape'='JAPE'
(Other definitions for jape that I've seen before include "Trick or prank" , "Jolly joke" , "Practical joke, jest" , "Kid" , "Trick - joke - jest" .)