A gas none may produce (4)
I believe the answer is:
neon
'a gas' is the definition.
(neon is a gas)
'none may produce' is the wordplay.
'may produce' indicates an anagram (I've seen 'might produce' mean this (I've seen this in other clues)).
'none' anagrammed gives 'NEON'.
(Other definitions for neon that I've seen before include "One of the six inert gases" , "None like bright city light" , "Gas used in fluorescent lamps" , "Gas used in fluorescent lighting" , "None (anag.)" .)