Gas ring area (5)
I believe the answer is:
ozone
'gas' is the definition.
(I've seen this before)
'ring area' is the wordplay.
'ring' becomes 'o'.
'area' becomes 'zone' (zone is a kind of area**).
'o'+'zone'='OZONE'
(Other definitions for ozone that I've seen before include "Kind of oxygen layer in atmosphere, has hole in it" , "Sea-air" , "Pungent form of oxygen" , "Sea air; bleaching agent" , "Gas forming protective layer" .)