Though only young, can still have a key (5)

I believe the answer is:

minor

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'a key' is the definition.
(thesaurus)

I don't understand the rest of the clue.

(Other definitions for minor that I've seen before include "Smaller - child" , "Insignificant" , "Like clubs" , "Paltry" , "Secondary" .)

I've seen this clue in the Evening Standard.

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