Zealot engaged in pop art is antagonistic (8)
I believe the answer is:
partisan
'zealot' is the definition.
(synonyms)
'engaged in pop art is antagonistic' is the wordplay.
'engaged in' indicates a hidden word.
'PARTISAN' can be found hidden inside 'pop art is antagonistic'.
(Other definitions for partisan that I've seen before include "champion" , "Old battle-axe" , "Prejudiced in favour of one side" , "Guerrilla" , "Strong party supporter" .)