Former student, briefly
I believe the answer is:
alum
'former student briefly' 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 "Colourless compound used in dyeing and tanning" , "Aluminium/potassium sulphate" , "Potassium compound; IoW sandy bay" , "Compound used in tanning and dying" , "Sulphate; maul (anag.)" .)