Heard any gas (4)

I believe the answer is:

neon

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'gas' is the definition.
(I've seen this before)

I don't understand the rest of the clue.

(Other definitions for neon that I've seen before include "illuminating agent" , "Colourless gas, used in lighting" , "Colourless, odourless rare gas" , "Form of lighting" , "Gas used in fluorescent lamps" .)

I've seen this clue in The Independent.

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