Mob returned British weapon (4)
I believe the answer is:
bomb
'weapon' is the definition.
(I know that bomb is a type of bomb)
'mob returned british' is the wordplay.
'returned' says the letters should be written in reverse.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'mob' back-to-front is 'bom'.
'bom'+'b'='BOMB'
(Other definitions for bomb that I've seen before include "See 18" , "Big bucks - flop" , "fortune" , "Exploding weapon" , "lot of money" .)