Deadly nerve gas
I believe the answer is:
sarin
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">deadly nerve gas</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 sarin that I've seen before include "Lethal nerve-gas" , "Gas used in Tokyo subway attack in 1995" , "Nerve gas - rains (anag)" .)