Nasty rogue, not a silly youth (9)
I believe the answer is:
youngster
'youth' is the definition.
(I've seen this before)
'nasty rogue not a silly' is the wordplay.
'not' indicates named letters should be taken away.
'silly' is an anagram indicator.
'nasty'+'rogue'='nastyrogue'
'nastyrogue' with 'a' taken out is 'nstyrogue'.
'nstyrogue' with letters rearranged gives 'YOUNGSTER'.
(Other definitions for youngster that I've seen before include "Rug rat in New York" , "tyke" , "Stripling" , "One not fully mature" , "Kid" .)