Fighting group losing its leader (6)
I believe the answer is:
action
'fighting' is the definition.
(action can mean action in war)
'group losing its leader' is the wordplay.
'group' becomes 'faction'.
'losing its leader' means to remove the first letter.
'faction' with its first letter removed is 'ACTION'.
(Other definitions for action that I've seen before include "Lawsuit; thing done" , "Deed - lawsuit - battle" , "Law suit" , "It is said to speak louder than words" , "Deed or legal proceeding" .)