Kind of layer (5)
I believe the answer is:
ozone
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">kind of layer</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for ozone that I've seen before include "Allotropic gas" , "Bracing air (colloq.)" , "(Depleted) layer of gas" , "That precious layer" , "Gas that screens ultraviolet radiation" .)