Highly addictive drug (6)
I believe the answer is:
heroin
'highly addictive drug' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for heroin that I've seen before include "Narcotic powder derived from morphine" , "Highly addictive morphine derivative" , "junk" , "Highly addictive narcotic" , "Highly addictive drug madefrom morphine" .)