Flanders river (4)
I believe the answer is:
yser
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">flanders 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 yser that I've seen before include "Glum" , "European flower" , "Belgian river associated with Flanders battleground" , "River -- battle scene in First World War" , "River rising in France" .)