Youngster of innate energy (4)

I believe the answer is:

teen

'youngster' is the definition.
(teenager)

'of innate energy' is the wordplay.
'of' says the answer is hidden in 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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