Medical or narcotic substance (4)
I believe the answer is:
drug
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">medical or narcotic substance</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)