Circle the area with the bracing air (5)
I believe the answer is:
ozone
'the bracing air' is the definition.
(I've seen this before)
'circle the area' is the wordplay.
'circle' becomes 'o'.
'the area' becomes 'zone' (zone is a kind of area).
'o'+'zone'='OZONE'
'with' acts as a link.
(Other definitions for ozone that I've seen before include "Sea air; bleaching agent" , "(Depleted) layer of gas" , "It's said to brace" , "Sea air (found in layer?)" , "Gas that screens ultraviolet radiation" .)