British crowd returned device for detonation (4)
I believe the answer is:
bomb
'device for detonation' is the definition.
(bombs are detonated)
'british crowd returned' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'crowd' becomes 'mob' (mob is a kind of crowd).
'returned' is a reversal indicator.
'b'+'mob'='bmob'
'bmob' reversed gives 'BOMB'.
(Other definitions for bomb that I've seen before include "Drop" , "worn-out car" , "One set off" , "One going off" , "Flop - load of money" .)