Indy setter essentially working on wordplay (4)
I believe the answer is:
punk
'indy setter' is the definition.
(Punk is a crossword setter for the Independent)
'essentially working on wordplay' is the wordplay.
'essentially' says to take the centre.
'on' says to put letters next to each other.
'wordplay' becomes 'pun' (synonyms).
The middle of 'working' is 'k'.
'k' put after 'pun' is 'PUNK'.
(Other definitions for punk that I've seen before include "Rock music -- American thug" , "Type of rock music" , "Worthless person; seventies rock style" , "Aggressive youth movement of the 1970s" , "Rebellious 1970s youth" .)