A really good firework! (7)

I believe the answer is:

cracker

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'firework' is the definition.
(cracker is a kind of firework)

I don't understand the remainder of the clue.

(Other definitions for cracker that I've seen before include "Christmas table item" , "pulled at Christmas?" , "Xmas table extra" , "Dry biscuit; Christmas item" , "Knockout biscuit?" .)

I've seen this clue in the Evening Standard.

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