Stratospheric layer (5)
I believe the answer is:
ozone
'stratospheric 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 "Form of atmospheric oxygen" , "Sea air; toxic gas" , "Gas forming protective layer" , "Sea-air" , "By sea, you could take in this" .)