Argon or xenon (3)

I believe the answer is:

gas

I believe this clue is a double definition.

'argon' is the first definition.
(thesaurus)

'xenon' is the second definition.
(thesaurus)

(Other definitions for gas that I've seen before include "Domestic fuel" , "Chat (colloq.)" , "Petrol (US)" , "What's needed to drive" , "16 (US)" .)

I've seen this clue in the King Feature Syndicate and the LA Times.

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