It's too late to be boyish! (5)

I believe the answer is:

manly

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'boyish' is the definition.
Both the answer and definition are adjectives. Perhaps you can see an association between them that I can't see?

I cannot understand how the rest of the clue works.

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(Other definitions for manly that I've seen before include "Masculine" , "Virile" , "With male attributes" , "Macho" , "Bold" .)

I've seen this clue in the Evening Standard.

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