None used gas (4)
I believe the answer is:
neon
'gas' is the definition.
(neon is a gas)
'none used' is the wordplay.
'used' is an anagram indicator.
'none' with letters rearranged gives 'NEON'.
(Other definitions for neon that I've seen before include "Gas used in fluorescent lighting" , "Gas used in fluorescent lamps" , "Inert gas, symbol Ne" , "Gaseous element, in advertising signs, say" , "Could it brighten proceedings" .)