Youngster given drug on lump of sugar? (4)
I believe the answer is:
cube
'lump of sugar?' is the definition.
(I've seen this before)
'youngster given drug' is the wordplay.
'youngster' becomes 'cub' (I've seen this before).
'given' means one lot of letters go next to another.
'drug' becomes 'e' (E is another name for ecstasy).
'cub'+'e'='CUBE'
'on' is the link.
(Other definitions for cube that I've seen before include "Product of three equal numbers" , "Power of three" , "Times itself twice" , "product of three factors" , "Shape made of six equal squares" .)