Gas round area (5)
I believe the answer is:
ozone
'gas' is the definition.
(I've seen this before)
'round area' is the wordplay.
'round' becomes 'o' (round can mean a round object).
'area' becomes 'zone' (zone is a kind of area**).
'o'+'zone'='OZONE'
(Other definitions for ozone that I've seen before include "Colourless gas soluble in alkalis and cold water" , "Shield against radiation" , "Less common form of oxygen" , "That precious layer" , "Form of oxygen in earth's upper atmosphere - there's a hole in it" .)