Historic British prep school (4)
I believe the answer is:
eton
'historic british prep school' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for eton that I've seen before include "College in Berkshire" , "Famous English public school" , "Posh English school of note" , "Easy to understand" , "Posh college" .)