One gas, or maybe none (4)
I believe the answer is:
neon
'one gas' is the definition.
(thesaurus)
'maybe none' is the wordplay.
'maybe' indicates anagramming the letters.
'none' anagrammed gives 'NEON'.
'or' acts as a link.
(Other definitions for neon that I've seen before include "None (anag.)" , "Element that gives a red glow in a vacuum tube" , "illuminating agent" , "Small plant; colour" , "Inert gas, symbol Ne" .)