Bracing air round region (5)
I believe the answer is:
ozone
'bracing air' is the definition.
(I've seen this in another clue)
'round region' is the wordplay.
'round' becomes 'o' (o is a circular letter).
'region' becomes 'zone' (zone is a kind of region).
'o'+'zone'='OZONE'
(Other definitions for ozone that I've seen before include "A gas; threatened layer in air" , "A poisonous gas; seaside air" , "Layer that protects against ultraviolet rays" , "Sea-air" , "Allotrope of oxygen" .)