Whippersnapper (9)
(Other definitions for youngster that I've seen before include "one of tender years" , "Juvenile" , "tyke" , "One not fully mature" , "Stripling" .)
'whippersnapper' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for whelp that I've seen before include "impudent young lad" , "Puppy, cub" , "Have puppies" , "Puppy- young wolf" , "Young pup" .)