Deadly substance (6)
I believe the answer is:
poison
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">deadly substance</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for poison that I've seen before include "Dangerous substance" , "It causes illness or death in a living organism" , "Meat for someone else" , "Eg, strychnine" , "Venom, toxin" .)