Colourless gas (5)
(Other definitions for argon that I've seen before include "Chemical element" , "noble element" , "Gas, 1% of atmosphere" , "Inert gas, symbol Ar" , "Noble gas, atomic number 18" .)
(Other definitions for xenon that I've seen before include "Heavy gaseous element" , "Noble element" , "Noble gas used in lamps" , "Inert gas used in radio valves" , "Inert 18" .)
'colourless gas' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for ethane that I've seen before include "Ate hen (anag) - hydrocarbon gas" , "Athene (anag.); a colourless gas" , "Flammable hydrocarbon" , "Fuel" , "Compound" .)
(Other definitions for butane that I've seen before include "Lighter fuel" , "Inflammable gas produced from petrol" , "Flammable gas in petroleum" , "Bottled gas used as a fuel" .)
(Other definitions for oxygen that I've seen before include "Element forming part of water and air" , "Gas that supports life" , "Gas forming about 20 per cent of atmosphere" , "The second most common element on earth" , "Abundant gas" .)