Chemical extracted from chalk -- a lime (6)

I believe the answer is:

alkali

'chemical' is the definition.
(thesaurus)

'extracted from chalk a lime' is the wordplay.
'extracted from' says the answer is hidden in the clue.
'ALKALI' can be found hidden inside 'chalk a lime'.

(Other definitions for alkali that I've seen before include "Eg, lime or caustic soda" , "Soluble compound that turns litmus paper blue" , "Compound turning litmus blue" , "Opposite of acid" , "Antacid" .)

I've seen this clue in The Telegraph.

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