It describes a group of youngsters between a dozen and a score (7)

I believe the answer is:

teenage

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'score' is the definition.
Both the answer and definition are adjectives. Perhaps there's a link between them I don't understand?

I can't explain the remainder of the clue.

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(Other definitions for teenage that I've seen before include "Gee, neat (anag)" , "Youthful" , "Eat gene to become young" , "Tea gene (anag.)" , "Adolescent" .)

I've seen this clue in The Globe and Mail.

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