Gas round the region (5)
I believe the answer is:
ozone
'gas' is the definition.
(I've seen this before)
'round the region' is the wordplay.
'round' becomes 'o' (round can mean a round object).
'the region' becomes 'zone' (zone is a kind of region).
'o'+'zone'='OZONE'
(Other definitions for ozone that I've seen before include "It's said to brace" , "Bracing air of old, now seen as potentially toxic" , "A gas; threatened layer in air" , "Stratospheric layer of gas -- invigorating sea air" , "Gas in upper atmosphere" .)