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' indicates putting letters inside.
'l' inserted into '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" .)