School memo written up (4)
I believe the answer is:
eton
'school' is the definition.
(famous UK public school)
'memo written up' is the wordplay.
'memo' becomes 'note' (I've seen this before).
'written up' says the letters should be written in reverse.
'note' reversed gives 'ETON'.
(Other definitions for eton that I've seen before include "College in Berkshire" , "Independent school" , "Famous college" , "Town on the Thames noted for its school" , "School near Slough" .)