Mark returns to school (4)
I believe the answer is:
eton
'school' is the definition.
(famous UK public school)
'mark returns' is the wordplay.
'mark' becomes 'note' ('note' can be a synonym of 'mark').
'returns' says the letters should be written in reverse.
'note' reversed gives 'ETON'.
'to' acts as a link.
(Other definitions for eton that I've seen before include "Public school founded in 1440" , "College in Berkshire" , "School - note (anag)" , "Berks college" , "collar, perhaps?" .)