Layer in the atmosphere (5)
I believe the answer is:
ozone
'layer in the atmosphere' 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 "Sea air (found in layer?)" , "Gas; bracing sea air" , "Kind of oxygen layer in atmosphere, has hole in it" , "Colourless gas, O3" , "Toxic form of oxygen" .)