Windsor Castle neighbor (4)
I believe the answer is:
eton
'windsor castle neighbor' 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 "School - note (anag)" , "Note sent up to English public school" , "School near Slough" , "collar, perhaps?" , "English college of note" .)