Form of oxygen (5)
I believe the answer is:
ozone
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">form of oxygen</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for ozone that I've seen before include "Invigorating air of the seaside" , "Toxic form of oxygen" , "Gas formed from oxygen" , "Seaside air, maybe" , "That precious layer" .)