Concerned with fighting response (8)
I believe the answer is:
reaction
'response' is the definition.
('reaction' can be a synonym of 'response')
'concerned with fighting' is the wordplay.
'concerned with' becomes 're' (**).
'fighting' becomes 'action' (I've seen this before).
're'+'action'='REACTION'
(Other definitions for reaction that I've seen before include "Chemical change or response" , "effect of drugs?" , "Reflex; backlash" , "Into care (anag)" , "Answer" .)