River in N France; scene of heavy fighting in World War One (5)

I believe the answer is:

somme

(Other definitions for somme that I've seen before include "French river known from World War 1" , "World War I battlefield around French river" , "First World War battlefield" , "French river, scene of 1916 battles" , "WW1 battlefield" .)

I've seen this clue in The Mirror.

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