Youngster knocking over girl’s pint (9)
I believe the answer is:
stripling
'youngster' is the definition.
(I know that stripling is a type of youngster)
'knocking over girl's pint' is the wordplay.
'knocking over' is an anagram indicator.
'girls'+'pint'='girlspint'
'girlspint' is an anagram of 'STRIPLING'.
(Other definitions for stripling that I've seen before include "Young 'un" , "Young boy" , "Young man" , "Lad" , "Youth" .)