Shop at anew to get the chemical substance (6)

I believe the answer is:

potash

(Other definitions for potash that I've seen before include "Powerful alkali used in gardening" , "Strong alkali derived from burnt wood" , "Potassium compound used in agriculture and industry" , "Type of fertiliser" , "Powerful alkali from burnt wood" .)

I've seen this clue in the Evening Standard.

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