Fighter fighting a dissenting group (7)
I believe the answer is:
faction
'a dissenting group' is the definition.
(I've seen this before)
'fighter fighting' is the wordplay.
'fighter' becomes 'F' (abbreviation in war plane e.g. F-16 etc.).
'fighting' becomes 'action' (action can mean action in war).
'f'+'action'='FACTION'
(Other definitions for faction that I've seen before include "Dissenting group - drama documentary" , "group of rebels" , "Cabal" , "Small united group within larger one" , "Seeing political dissenters" .)