Fresh bracing air (5)
I believe the answer is:
ozone
'fresh bracing air' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for ozone that I've seen before include "Pungent form of oxygen" , "Form of atmospheric oxygen" , "Sea air (found in layer?)" , "Bracing seaside air" , "Gas; bracing sea air" .)