Fighting against (4)
I believe the answer is:
anti
'fighting against' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for anti that I've seen before include "An opponent - opposed to party policy, etc" , "'No' voter" , "Someone opposed (to a policy?)" , "on the other side" , "(Person who is) not in favour" .)