Crossword Genius

Could be a flute or a balloon (5)

Ross

I believe the answer is:

glass

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'a balloon' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Maybe there's an association between them 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 "such as one might hold at party" , "Brittle transparent material" , "(Material used in) mirror" , "Transparent stuff" , "that one can see through" .)

I've seen this clue in the Evening Standard.
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