Ring area supplying gas (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'
'supplying' acts as a link.
(Other definitions for ozone that I've seen before include "Layer that protects against ultraviolet rays" , "Sea-air" , "Allotropic gas" , "Stratospheric layer of gas -- invigorating sea air" , "Colourless gas, O3" .)