Narcotic, for example (4)
I believe the answer is:
drug
'narcotic for example' is the definition.
(narcotic is a kind of drug)
This is all the clue.
(Other definitions for drug that I've seen before include "Tranquillize" , "Medicinal substance" , "Item of medicine or a narcotic" , "Sedate" , "may be soft or hard" .)