Boris Johnson's alma mater
I believe the answer is:
eton
'boris johnson's alma mater' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for eton that I've seen before include "Famous college" , "Town opposite Windsor on the Thames" , "Noted public school" , "Public school, established in 1440" , "collar, perhaps?" .)