Nothing stops just fight (4)
I believe the answer is:
bout
'fight' is the definition.
('bout' can be a synonym of 'fight')
'nothing stops just' is the wordplay.
'nothing' becomes 'o' (looks like zero - 0).
'stops' is an insertion indicator.
'just' becomes 'but' (I've seen this in other clues).
'o' going into 'but' is 'BOUT'.
(Other definitions for bout that I've seen before include "(Drinking) session" , "Wrestling contest" , "Contest between boxers" , "Spell of eg drinking" , "Spell - boxing match" .)