Bar 40 cutting a drug (4)
I believe the answer is:
axle
'bar' is the definition.
(I've seen this before)
'40 cutting a drug' is the wordplay.
'40' becomes 'xl' (roman numeral).
'cutting' means one lot of letters goes inside another (some letters cut their way into a word).
'drug' becomes 'e' (e is another name for ecstasy).
'a'+'e'='ae'
'xl' placed into 'ae' is 'AXLE'.
(Other definitions for axle that I've seen before include "Rod attachment to wheels" , "Shaft with wheels attached" , "Rod supporting wheels" , "Wheel spindle" , "Rod at centre of wheel" .)