Youngster has wrong rig left (4)
I believe the answer is:
girl
'youngster' is the definition.
'wrong rig left' is the wordplay.
'wrong' is an anagram indicator.
'left' becomes 'L' (common abbreviation).
'rig' with letters rearranged gives 'gir'.
'gir'+'l'='GIRL'
'has' acts as a link.
(Other definitions for girl that I've seen before include "Maidservant" , "Young woman" , "Lass" , "Young lady" , "A young female" .)