Youngster of infinite energy (4)
I believe the answer is:
teen
'youngster' is the definition.
(I've seen this in another clue)
'of infinite energy' is the wordplay.
'of' indicates the answer is hidden within the clue.
'TEEN' can be found hidden inside 'infinite energy'.
(Other definitions for teen that I've seen before include "Youngster of a certain age" , "Juvenile" , "Adolescent (informal)" , "Youth" , "Of 13 to 19 year olds" .)