Styptic pencil stuff (4)
I believe the answer is:
'styptic pencil stuff' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for alum that I've seen before include "Compound used in tanning and dying" , "Sulphate; maul (anag.)" , "double sulphate" , "Name given to a specific chemical compound and a class of them" , "Colourless compound used in dyeing and tanning" .)