Youngster left, removed from nightspot (3)
I believe the answer is:
'youngster' is the definition.
(cub can mean a young person)
'left removed from nightspot' is the wordplay.
'left' becomes 'l' (common abbreviation).
'removed from' is a deletion indicator.
'nightspot' becomes 'club' (I've seen this in another clue).
'club' with 'l' removed is 'CUB'.
(Other definitions for cub that I've seen before include "Young lion or fox" , "'Baby animal, fox say (3)'" , "Young creature" , "Young one" , "Novice" .)