Colourless gas, O3 (5)
I believe the answer is:
ozone
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">colourless gas</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >'<span class="explanation-format__original">o3</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ ></span>
(Other definitions for ozone that I've seen before include "Sea air; toxic gas" , "Sea air (found in layer?)" , "Toxic form of oxygen" , "Atmospheric layer in some danger" , "Gas; bracing sea air" .)