Gas industry talk starting late (5)

I believe the answer is:

argon

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

I don't understand the remainder of the clue.

(Other definitions for argon that I've seen before include "Gas, inert constituent of air" , "rare gas" , "something inert and insubstantial" , "17 [GAS]" , "Inert gas, present in small amounts in the air" .)

I've seen this clue in The Times.

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