Circular belt of gas (5)
I believe the answer is:
ozone
'gas' is the definition.
(I know that ozone is a type of gas)
'circular belt' is the wordplay.
'circular' becomes 'o' (I've seen this in other clues).
'belt' becomes 'zone' (I've seen this before).
'o'+'zone'='OZONE'
'of' is the link.
(Other definitions for ozone that I've seen before include "invigorating British air" , "Bracing seaside air" , "Gas that screens ultraviolet radiation" , "Allotropic gas" , "A poisonous gas; seaside air" .)