Balloon gas (6)

I believe the answer is:

helium

'balloon gas' is the definition.
(I've seen this in another clue)

This is all the clue.

(Other definitions for helium that I've seen before include "Balloon-filling element" , "Light colourless gas used in airships" , "Second lightest element" , "Lightest noble gas" , "Light colourless gas used in balloons" .)

I've seen this clue in The Mirror, the LA Times, The Daily Mail, the Evening Standard and the King Feature Syndicate.

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