Extract of chalk, a limestone, would be this (6)
I believe the answer is:
alkali
'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.
Can you help me to learn more?
(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" .)