Crossword Genius

Is not a volatile shareholding? (5,5)

Ross

I believe the answer is:

piano stool

I'm a little stuck... Click here to teach me more about this clue!

'volatile shareholding?' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

'is not a' is the wordplay.
I cannot really understand how this works, but
'not' could be 'no' (I've seen this in other clues) and 'no' is found within the answer.
'a' is found in the answer.
'is' is present in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

(Another definition for piano stool that I've seen is " Player's support".)

I've seen this clue in the Sydney Morning Herald.
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