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' (I've seen this in other clues).
'out' is an insertion indicator.
'l' placed into '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" .)