Stratospheric gas (5)
I believe the answer is:
ozone
'stratospheric gas' 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 "Pungent form of oxygen" , "Invigorating air of the seaside" , "Stratospheric layer of gas -- invigorating sea air" , "Gas forming screen against ultraviolet radiation in the stratosphere" , "Condensed oxygen; sea air" .)