Gas — none spreading (4)
I believe the answer is:
neon
'gas' is the definition.
(thesaurus)
'none spreading' is the wordplay.
'spreading' is an anagram indicator.
'none' is an anagram of 'NEON'.
(Other definitions for neon that I've seen before include "Gaseous element, in advertising signs, say" , "Element 10, used in lights" , "Gas - light" , "Colourless, odourless rare gas" , "Gas used in fluorescent lamps" .)