Kind of layer (5)
I believe the answer is:
ozone
'kind of 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 "Allotropic gas" , "Bracing air (colloq.)" , "(Depleted) layer of gas" , "That precious layer" , "Gas that screens ultraviolet radiation" .)