Upper atmospheric layer (5)
I believe the answer is:
ozone
'upper atmospheric layer' is the definition.
(I've seen this in another clue)
This is all the clue.
(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" .)