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' (young bear).
'out' means one lot of letters goes inside another.
'l' inserted within '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" .)