Gas that none used (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' anagrammed gives 'NEON'.
'that' acts as a link.
(Other definitions for neon that I've seen before include "None like bright city light" , "Gas - light" , "Colourless, odourless rare gas" , "It may illuminate" , "Sign gas" .)