Carry drug left by groups in collusion (6)
I believe the answer is:
cartel
'groups in collusion' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'carry drug left' is the wordplay.
'carry' becomes 'cart' (carting is a kind of carrying).
'drug' becomes 'e' (e is another name for ecstasy).
'left' becomes 'l' (common abbreviation).
'cart'+'e'+'l'='CARTEL'
'by' acts as a link.
Can you help me to learn more?
(Other definitions for cartel that I've seen before include "Protective trade group for claret" , "Price-fixing association" , "Market-controlling arrangement" , "Business group" , "Claret (anag)" .)