If damp it's unlikely to make much impact (5)

I believe the answer is:

squib

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'impact' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for squib that I've seen before include "Ultimately explosive" , "little banger" , "Lampoon; firework" , "piece of satire" , "Firework (disappointing, if damp)" .)

I've seen this clue in The Globe and Mail.

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