Capacity of youngster to mature (6)

I believe the answer is:

cubage

'capacity' is the definition.
(I've seen this before)

'youngster to mature' is the wordplay.
'youngster' becomes 'cub' (young bear).
'to mature' becomes 'age' (synonyms).
'cub'+'age'='CUBAGE'

'of' acts as a link.

(Other definitions for cubage that I've seen before include "Content" , "capacity" .)

I've seen this clue in The Guardian.

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