Mannish in a youthful way? (6)

I believe the answer is:

boyish

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'mannish' is the definition.
Both the answer and definition are adjectives. Perhaps they are linked in a way I don't understand?

I can't explain the rest of the clue.

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(Other definitions for boyish that I've seen before include "Juvenile" , "Like a young lad" , "Youthful" , "like a young man" , "how immature youth is" .)

I've seen this clue in the Evening Standard.

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