Grad (4)
I believe the answer is:
'grad' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for alum that I've seen before include "Sulphate; maul (anag.)" , "Aluminium/potassium sulphate" , "Colourless compound used in dyeing and tanning" , "Name given to a specific chemical compound and a class of them" , "Potassium compound; IoW sandy bay" .)