One of several kids; between a dozen and a score (8)

I believe the answer is:

teenager

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'one' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.

I don't understand how the remainder of the clue works.

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(Other definitions for teenager that I've seen before include "Youngster" , "one that's not yet made a score?" , "14, perhaps" , "youth, perhaps" , "Generate (anag)" .)

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

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