Left a youngster out of the group (4)
I believe the answer is:
club
'the group' is the definition.
The answer is an example.
'left a youngster out' is the wordplay.
'left' becomes 'l' (common abbreviation).
'a youngster' becomes 'cub' (cub can mean a young person).
'out' is an insertion indicator.
'l' placed within 'cub' is 'CLUB'.
'of' acts as a link.
(Other definitions for club that I've seen before include "woman expects to be in" , "Iron or putter, say" , "may give members benefits" , "'Short stick, card suit and association (4)'" , "Truncheon" .)