Colourless double sulphate of aluminium and potassium used in dressing leather (4)

I believe the answer is:

alum

(Other definitions for alum that I've seen before include "Colourless compound used in dyeing and tanning" , "Sulphate; maul (anag.)" , "double sulphate" , "Compound used in tanning and dying" , "Potassium compound; IoW sandy bay" .)

I've seen this clue in The Mirror.

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