London's river (6)
I believe the answer is:
thames
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">london's river</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for thames that I've seen before include "Longest English river" , "capital banker" , "Capital's river" , "The Ma's in the river" , "The Ma's in London" .)