Left a youngster out of society (4)
I believe the answer is:
club
'society' is the definition.
(synonyms)
'left a youngster out' is the wordplay.
'left' becomes 'l' (common abbreviation).
'a youngster' becomes 'cub' (I've seen this in other clues).
'out' is an insertion indicator.
'l' inserted into 'cub' is 'CLUB'.
'of' acts as a link.
(Other definitions for club that I've seen before include "sort of sandwich" , "'Short stick, card suit and association (4)'" , "Union - strike" , "Place for dance" , "Weapon; playing card" .)