Youth of innate energy (4)
I believe the answer is:
teen
'youth' is the definition.
(thesaurus)
'of innate energy' is the wordplay.
'of' indicates a hidden word.
'TEEN' is hidden within 'innate energy'.
(Other definitions for teen that I've seen before include "Adolescent, then adult-to-be" , "Youngster of a certain age" , "Adolescent (informal)" , "Juvenile" , "Of 13 to 19 year olds" .)