River at York (4)
I believe the answer is:
ouse
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">river at york</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for ouse that I've seen before include "Flowing water" , "Great river at King's Lynn" , "Name of several English rivers" , "Runner" , "bird has lost its tail" .)