Upper atmospheric layer (5)
I believe the answer is:
ozone
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">upper atmospheric layer</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 ozone that I've seen before include "That precious layer" , "Gas; bracing sea air" , "Form of oxygen in earth's upper atmosphere - there's a hole in it" , "Toxic form of oxygen" , "lively atmosphere" .)