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Extract of chalk, a limestone, would be this (6)

Ross

I believe the answer is:

alkali

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

'extract of chalk a limestone would' is the wordplay.
'extract of' indicates a hidden word (I've seen this in other clues).
'ALKALI' is hidden in the letters of 'chalk a limestone would'.

'be' acts as a link.

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(Other definitions for alkali that I've seen before include "Substance such as lime or caustic soda" , "Non-acid" , "Chemical base" , "In pH terms, not acid" , "Antacid" .)

I've seen this clue in The Times.
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