Endowment donor, often (4)
I believe the answer is:
alum
'endowment donor often' 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 "Compound used in tanning and dying" , "White mineral salt used in medicine and in dyeing" , "Aluminium/potassium sulphate" , "Sulphate; maul (anag.)" , "Potassium compound; IoW sandy bay" .)