Opponent pawns this occasionally (4)
I believe the answer is:
anti
'opponent' is the definition.
(someone opposed to something)
'pawns this occasionally' is the wordplay.
'occasionally' means one should take alternating letters (letters taken at regular intervals or occasions).
'pawns'+'this'='pawnsthis'
The alternating letters of 'pawnsthis' are 'ANTI'.
(Other definitions for anti that I've seen before include "Policy's opponent" , "[NO MAN]" , "one opposing?" , "Hostile" , "Who pooh-poohs all" .)