Crossword Genius

Booster, perhaps (4)

Ross

I believe the answer is:

alum

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

I've seen this clue in The New York Times.
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