Crossword Genius

Reunion attendee, casually (4)

Ross

I believe the answer is:

alum

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

I've seen this clue in The Wall Street Journal.
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