Former drug mixture (5)
I believe the answer is:
paste
'mixture' is the definition.
(paste is a kind of mixture)
'former drug' is the wordplay.
'former' becomes 'past' ('past' can be similar in meaning to 'former').
'drug' becomes 'e' (e is another name for ecstasy).
'past'+'e'='PASTE'
(Other definitions for paste that I've seen before include "It is used to stick wallpaper" , "foie gras" , "Sticky stuff" , "Creamy, sticky, or doughy mixture" , "Adhesive gum" .)