Fighting back is natural (3)
I believe the answer is:
raw
'natural' is the definition.
(I know that natural can be written as raw)
'fighting back' is the wordplay.
'fighting' becomes 'war'.
'back' shows that the letters should be reversed in order.
'war' reversed gives 'RAW'.
'is' is the link.
(Other definitions for raw that I've seen before include "not yet ready to eat" , "Damp and chilly" , "Natural" , "Uncooked or crude" , "Uncooked or unsophisticated" .)