Poisonous substance (5)
I believe the answer is:
toxin
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">poisonous substance</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for toxin that I've seen before include "Form of poison" , "Poison, venom" , "Poisonous substance" , "Poison of organic origin" , "Natural poison" .)