Youngster of innate energy (4)

I believe the answer is:

teen

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

'of innate energy' is the wordplay.
'of' indicates the answer is hidden within the clue.
'TEEN' is hidden in the letters of 'innate energy'.

(Other definitions for teen that I've seen before include "Youngster of a certain age" , "Adolescent, then adult-to-be" , "Young person" , "Youngster over twelve" , "Adolescent (informal)" .)

I've seen this clue in The Mirror.

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