Medical or narcotic substance (4)
I believe the answer is:
drug
'medical or narcotic substance' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for drug that I've seen before include "may be soft or hard" , "Maybe addictive substance" , "Tranquillize" , "Substance used in medicine, often abused" , "Substance used as medicine or narcotic" .)