Toxic form of oxygen (5)
I believe the answer is:
ozone
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">toxic form of oxygen</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 ozone that I've seen before include "Trioxygen" , "something found in the atmosphere" , "Toxic form of oxygen" , "Fresh, bracing sea air" , "Gas forming protective layer" .)