Gangsters upset over British weapon (4)
I believe the answer is:
bomb
'weapon' is the definition.
(I have seen 'Explosive weapon ' mean 'bomb' so perhaps 'weapon' could also mean 'bomb')
'gangsters upset over british' is the wordplay.
'gangsters' becomes 'mob' (mob can mean the Mafia).
'upset over' is a reversal indicator (knocked over).
'british' becomes 'b' (abbreviation e.g. in BBC).
'mob' in reverse letter order is 'bom'.
'bom'+'b'='BOMB'
(Other definitions for bomb that I've seen before include "See 18" , "Drop" , "move quickly" , "worn-out car" , "Explosive device" .)