Argue violently about evil fighter (8)
I believe the answer is:
guerilla
'fighter' is the definition.
(I know that guerilla is a type of fighter)
'argue violently about evil' is the wordplay.
'violently' indicates an anagram (letters are violently disrupted).
'about' means one lot of letters goes inside another.
'evil' becomes 'ill' (as in doing people ill).
'argue' with letters rearranged gives 'guera'.
'guera' placed around 'ill' is 'GUERILLA'.
(Other definitions for guerilla that I've seen before include "Freedom fighter" , "partisan" , "He fights as part of small independent group" , "Member of a small independent armed force" , "I'll fight irregularly" .)