Oppose or fight against (6)
I believe the answer is:
resist
This clue is a double definition.
'oppose' is the first definition.
(resisting is a kind of opposing)
'fight against' is the second definition.
(I've seen this before)
(Other definitions for resist that I've seen before include "Forgo" , "Go against" , "Combat" , "Hold out for sister" , "Defy for sister" .)