Atmosphere layer (5)
I believe the answer is:
ozone
'atmosphere layer' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ozone that I've seen before include "(Depleted) layer of gas" , "Atmospheric layer in some danger" , "Layer that protects against ultraviolet rays" , "It's said to brace" , "Colourless gas soluble in alkalis and cold water" .)