___ Bay, Isle of Wight tourist spot, famous for its multi-coloured sand cliffs (4)

(Other definitions for alum that I've seen before include "Colourless compound used in dyeing and tanning" , "Compound used in dyeing and to stop bleeding" , "Compound used in tanning and dying" , "White mineral salt used in medicine and in dyeing" , "Name given to a specific chemical compound and a class of them" .)