Last year's senior, briefly (4)

I believe the answer is:

alum

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

I've seen this clue in the Penny Press.

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