Invigorating air round region (5)
I believe the answer is:
ozone
'invigorating air' is the definition.
(I've seen this before)
'round region' is the wordplay.
'round' becomes 'o' (round can mean a round object).
'region' becomes 'zone' (zone is a kind of region).
'o'+'zone'='OZONE'
(Other definitions for ozone that I've seen before include "Layer that protects against ultraviolet rays" , "Less common form of oxygen" , "something refreshing" , "Bracing air (colloq.)" , "Bracing air of old, now seen as potentially toxic" .)