Maybe helium's left inside balloon? (5)

I believe the answer is:

glass

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'balloon?' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe they are linked in a way I don't understand?

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for glass that I've seen before include "Balloon, say" , "Vitreous material" , "Eg, tumbler" , "Perhaps schooner" , "Translucent, usually transparent, material" .)

I've seen this clue in The Independent.

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