Gas round a region (5)
I believe the answer is:
ozone
'gas' is the definition.
(I know that ozone is a type of gas)
'round a region' is the wordplay.
'round' becomes 'o' (round can mean a round object).
'a region' becomes 'zone' (zone is a kind of region).
'o'+'zone'='OZONE'
(Other definitions for ozone that I've seen before include "Colourless gas soluble in alkalis and cold water" , "Bracing air of old, now seen as potentially toxic" , "Sea air; bleaching agent" , "Gas that screens ultraviolet radiation" , "Layer that protects against ultraviolet rays" .)