Alma mater visitor (4)
I believe the answer is:
'alma mater visitor' 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 "Name given to a specific chemical compound and a class of them" , "Compound used in tanning and dying" , "double sulphate" , "Potassium compound; IoW sandy bay" , "Sulphate used in dyeing and tanning" .)