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' (cub can mean a young person).
'out' means one lot of letters goes inside another.
'l' put inside 'cub' is 'CLUB'.
'of' is the 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" .)