Fresh air (5)
I believe the answer is:
ozone
'fresh air' is the definition.
(ozone can informally mean fresh air)
This is the entire clue.
(Other definitions for ozone that I've seen before include "Stratospheric layer of gas -- invigorating sea air" , "Colourless gas, O3" , "Ring" , "Gas forming screen against ultraviolet radiation in the stratosphere" , "Layer that protects against ultraviolet rays" .)