Atmospheric layer (5)
'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 "bracing atmosphere" , "Stratospheric layer of gas -- invigorating sea air" , "That precious layer" , "Layer that protects against ultraviolet rays" , "Gas in upper atmosphere" .)
'atmospheric layer' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for stratosphere that I've seen before include "Layer of high air around the earth" , "Part of the air" , "Zone high above earth" , "Region about ten to fifty km above the earth" , "Atmospheric layer" .)