Juvenile delinquent stoner guy (9)
I believe the answer is:
youngster
'juvenile' is the definition.
(I know that child is a type of juvenile)
'delinquent stoner guy' is the wordplay.
'delinquent' indicates an anagram (I've seen this in other clues).
'stoner'+'guy'='stonerguy'
'stonerguy' is an anagram of 'YOUNGSTER'.
(Other definitions for youngster that I've seen before include "Youth" , "Kid" , "Adolescent" , "Stripling" , "Tiddler" .)