Dismiss something brilliant? 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 something brilliant?' is the wordplay.
I cannot really see how this works, but
'dismiss' could be 'fire' (synonyms) and 'fire' is found within the answer.
'brilliant?' could be 'crack' (I've seen this before) and 'crack' is located in the answer.
The remaining letters 'er' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Another definition for firecracker that I've seen is " banger".)