Double sulphate of aluminium and potassium (4)

I believe the answer is:

alum

(Other definitions for alum that I've seen before include "Compound used in dyeing and to stop bleeding" , "White mineral salt used in medicine and in dyeing" , "Aluminium/potassium sulphate" , "Sulphate; maul (anag.)" , "Colourless compound used in dyeing and tanning" .)

I've seen this clue in The Telegraph.

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