School fundraiser target, informally (4)

I believe the answer is:

alum

(Other definitions for alum that I've seen before include "Sulphate used in dyeing and tanning" , "Potassium compound; IoW sandy bay" , "double sulphate" , "Compound used in dyeing and to stop bleeding" , "Name given to a specific chemical compound and a class of them" .)

I've seen this clue in the Universal.

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