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Old drip's a washout if it's damp (5)

Ross

I believe the answer is:

squib

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

I don't understand the remainder of the clue.

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(Other definitions for squib that I've seen before include "Lampoon; firework" , "Firework (disappointing, if damp)" , "piece of satire" , "Jumping firework" , "Firework (perhaps damp!)" .)

I've seen this clue in The Independent.
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