Gas from uncontained fart expanded to the max? (5)

I believe the answer is:

argon

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'gas' is the definition.
(argon is a gas)

I don't understand the rest of the clue.

(Other definitions for argon that I've seen before include "elementary" , "Inert gas, present in small amounts in the air" , "Noble gas, atomic number 18" , "Inert gas in atmosphere" , "17 [GAS]" .)

I've seen this clue in The Independent.

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