Dismiss solver? One may be let off (11)

I believe the answer is:

firecracker

'one may be let off' is the definition.
(I know that firecracker are often acted on by the action let off)

'dismiss solver?' is the wordplay.
'dismiss' becomes 'fire' (synonyms).
'solver?' becomes 'cracker' (someone who cracks things).
'fire'+'cracker'='FIRECRACKER'

(Another definition for firecracker that I've seen is " banger".)

I've seen this clue in The Times.

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