Youngster of acute energy (4)

I believe the answer is:

teen

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

'of acute energy' is the wordplay.
'of' says the answer is hidden in the clue.
'TEEN' is hidden in the letters of 'acute energy'.

(Other definitions for teen that I've seen before include "Youth" , "Youngster over twelve" , "Adolescent, then adult-to-be" , "Juvenile" , "Of 13 to 19 year olds" .)

I've seen this clue in The Mirror.

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