Heroin, for one (6)
I believe the answer is:
opiate
'heroin for one' is the definition.
(I know that diacetylmorphine is a type of opiate)
This is the entire clue.
(Other definitions for opiate that I've seen before include "Drug such as morphine" , "drug to aid rest" , "Drug with an opium base" , "Drug inducing a false sense of contentment" , "A drug to induce sleep" .)