Youngster in society lacking length (3)
I believe the answer is:
cub
'youngster' is the definition.
(I've seen this before)
'society lacking length' is the wordplay.
'society' becomes 'club' ('club' can be a synonym of 'society').
'lacking' suggests deleting specific letters.
'length' becomes 'l'.
'club' with 'l' taken out is 'CUB'.
'in' acts as a link.
(Other definitions for cub that I've seen before include "A baby fox or bear" , "Young animal or scout aspirant" , "Inexperienced reporter" , "'Baby animal, fox say (3)'" , "Young of certain animals" .)