Harrow rival (4)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">harrow rival</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for eton that I've seen before include "Public school (sounds like 20)" , "academic institution" , "Note sent up to English public school" , "Town opposite Windsor on the Thames" , "Public school near Windsor" .)