School on the Thames (4)
I believe the answer is:
'school on the thames' 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; jacket type" , "Posh college" , "Posh English school of note" , "Town opposite Windsor on the Thames" , "Berks college" .)