Famous college returned memo (4)
I believe the answer is:
note
'memo' is the definition.
(I've seen this before)
'famous college returned' is the wordplay.
'famous college' becomes 'eton' (I've seen this before).
'returned' is a reversal indicator.
'eton' reversed gives 'NOTE'.
(Other definitions for note that I've seen before include "Fiver, say" , "Eminence" , "Be mindful of" , "A, B, C, D, F or G" , "Mark - sound - money" .)