One drawn to serve apprenticeship? (7)

I believe the answer is:

trainee

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'one' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.

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

Can you help me to learn more?

(Other definitions for trainee that I've seen before include "novice" , "Apprentice" , "21 [TYRO]" , "encountered bad weather in driving area" , "Person under instruction" .)

I've seen this clue in the Evening Standard.

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