Back bitter conflict (3)
I believe the answer is:
war
'conflict' is the definition.
(war is a kind of conflict)
'back bitter' is the wordplay.
'back' says the letters should be written in reverse.
'bitter' becomes 'raw'.
'raw' written backwards gives 'WAR'.
(Other definitions for war that I've seen before include "Armed hostility" , "series of engagements" , "State of armed conflict" , "Fight" , "Major conflict" .)