Berkshire public school (4)
I believe the answer is:
eton
'berkshire public school' 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 "Posh college" , "Town on the Thames noted for its school" , "Public school near Windsor" , "Famous English public school" , "Public school (sounds like 20)" .)