Big city fails: if let off, they'll be fuming! (5,5)
I believe the answer is:
'if let off they'll be fuming' is the definition.
(smoke bombs give off fumes)
'big city fails' is the wordplay.
'big city' becomes 'smoke' ('the big smoke' can mean London).
'fails' becomes 'bombs' ('bomb' can be a synonym of 'fail').
I've seen this clue in The Times.
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