Regarding what you say, is not one of the listeners

I think it could be either:

Which length description are you looking for?:

3-7

10*

lipreading

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'of the listeners' is the definition.
The definition suggests a singular noun which matches the answer.

'regarding what you say is not one' is the wordplay.
I cannot quite see how this works, but
'regarding' could be 're' (re can mean regarding, concerning) and 're' is present in the answer.
'one' could be 'li' (I've seen this in other clues) and 'li' is found in the answer.
This explanation may well be incorrect...

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I've seen this clue in the Evening Standard.

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