Type of sandwich, large, fed to youngster (4)
I believe the answer is:
club
'type of sandwich' is the definition.
'large fed to youngster' is the wordplay.
'large' becomes 'l'.
'fed to' means one lot of letters goes inside another.
'youngster' becomes 'cub' (I've seen this in other clues).
'l' placed within 'cub' is 'CLUB'.
(Other definitions for club that I've seen before include "Dance venue" , "Stout stick, card suit and association" , "Place to meet one's friends with a weapon" , "Society - weapon" , "Good meeting-place" .)