Gas ring needs a belt (5)
I believe the answer is:
ozone
'gas' is the definition.
(I've seen this before)
'ring needs a belt' is the wordplay.
'ring' becomes 'o'.
'needs' is a charade indicator (letters next to each other).
'a belt' becomes 'zone' (I've seen this before).
'o'+'zone'='OZONE'
(Other definitions for ozone that I've seen before include "Atmospheric layer in some danger" , "Gas forming protective layer" , "Trioxygen" , "A gas; threatened layer in air" , "That precious layer" .)