Devotee into pop art is a numpty (8)
I believe the answer is:
partisan
'devotee' is the definition.
(I've seen this before)
'into pop art is a numpty' is the wordplay.
'into' indicates the answer is hidden within the clue.
'PARTISAN' can be found hidden inside 'pop art is a numpty'.
(Other definitions for partisan that I've seen before include "Prejudiced towards a particular cause" , "Guerrilla" , "Highly biased" , "Devotee of a cause" , "of prejudiced views" .)