Youth of infinite energy (4)
I believe the answer is:
teen
'youth' is the definition.
(teenager)
'of infinite energy' is the wordplay.
'of' says the answer is hidden in the clue.
'TEEN' is hidden in the letters of 'infinite energy'.
(Other definitions for teen that I've seen before include "Youth" , "Of 13 to 19 year olds" , "Juvenile" , "Adolescent, then adult-to-be" , "Youngster over twelve" .)