London river (6)
I believe the answer is:
thames
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">london river</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for thames that I've seen before include "It flows through London" , "River at Runnymede" , "The Ma's in London" , "Windsor's river" , "London's river (6)" .)