Memo causing upset to college (4)
I believe the answer is:
note
'memo' is the definition.
(I've seen this before)
'upset to college' is the wordplay.
'upset' is a reversal indicator.
'to college' becomes 'eton' (Eton College, English private school).
'eton' written backwards gives 'NOTE'.
'causing' is the link.
(Other definitions for note that I've seen before include "register" , "Observe the musical pitch." , "Fiver, say" , "Observe in Eton" , "Mark - sound - money" .)