Older youngster (4)
I believe the answer is:
teen
'older youngster' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for teen that I've seen before include "Of 13 to 19 year olds" , "Youngster over twelve" , "Adolescent (informal)" , "Adolescent, then adult-to-be" , "Youngster of a certain age" .)