Flanders river (4)
I believe the answer is:
yser
'flanders river' is the definition.
(I've seen this in another clue)
This is all the clue.
(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" .)