Paint this regularly for an opponent (4)
I believe the answer is:
anti
'an opponent' is the definition.
(thesaurus)
'paint this regularly' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'paint'+'this'='paintthis'
The alternating letters of 'paintthis' are 'ANTI'.
'for' is the link.
(Other definitions for anti that I've seen before include "Opposition member" , "Hostile" , "Adversary" , "Person opposed" , "Negative vote" .)