Rebellious group loudly fighting (7)
I believe the answer is:
faction
'rebellious group' is the definition.
(I've seen this before)
'loudly fighting' is the wordplay.
'loudly' becomes 'f' (short for forte - 'loud' in music).
'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" , "A dissenting clique" , "group of rebels" , "Act on if for a small group within a larger one" , "Clique that seeks power often through intrigue" .)