Not too old for winks, let's hope? (5)

I believe the answer is:

forty

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'not too old' is the definition.
Both the definition and answer are adjectives. Perhaps you can see a link between them that I can't see?

I don't understand the rest of the clue.

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(Other definitions for forty that I've seen before include "Two-score" , "number of penitential days" , "Two score" , "Kip" , "When life begins" .)

I've seen this clue in the Evening Standard.

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