Old drip's a washout if it's damp (5)
I believe the answer is:
squib
'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.
Can you help me to learn more?
(Other definitions for squib that I've seen before include "Lampoon; firework" , "Firework (disappointing, if damp)" , "piece of satire" , "Jumping firework" , "Firework (perhaps damp!)" .)