Gas one converted, nitrogen (4)
I believe the answer is:
neon
'nitrogen' is the definition.
(thesaurus)
'gas one converted' is the wordplay.
'gas' becomes 'N' (chemical symbol for nitrogen).
'converted' indicates an anagram.
'one' with letters rearranged gives 'eon'.
'n'+'eon'='NEON'
(Other definitions for neon that I've seen before include "Type of lighting" , "None (anag.)" , "One of the six inert gasses" , "It may illuminate" , "Colourless gas, used in lighting" .)