River of Flanders (4)
I believe the answer is:
'river of flanders' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for yser that I've seen before include "European river" , "River -- battle scene in First World War" , "Belgian river associated with Flanders battleground" , "River rising in France" , "European flower" .)