Memos about French river (5)
I believe the answer is:
somme
'french river' is the definition.
(the Somme is a river in France)
'memos about' is the wordplay.
'about' indicates an anagram.
'memos' with letters rearranged gives 'SOMME'.
(Other definitions for somme that I've seen before include "bloody fight" , "French 2 down" , "runner" , "River of northern France" , "Scene of WW One battles" .)