Fighting Bill in boxing ring (6)
I believe the answer is:
action
'fighting' is the definition.
(action can mean action in war)
'bill in boxing ring' is the wordplay.
'bill' becomes 'act' (an Act of Parliament).
'boxing' is an insertion indicator ('box' can mean contain, as in to put in a box).
'ring' becomes 'O' (letter resembling a ring).
'in' placed around 'o' is 'ion'.
'act'+'ion'='ACTION'
(Other definitions for action that I've seen before include "Combat" , "A judicial proceeding brought by one party against another" , "''. . . . . . speaks louder than words'' (6)" , "Military engagement" , "exploit" .)