Frenchman among several in French river (5)
I believe the answer is:
somme
'french river' is the definition.
(the Somme is a river in France)
'frenchman among several' is the wordplay.
'frenchman' becomes 'm' (abbreviation for Monsieur - like 'Mr').
'among' indicates putting letters inside.
'several' becomes 'some' (similar in meaning).
'm' put inside 'some' is 'SOMME'.
'in' is the link.
(Other definitions for somme that I've seen before include "Scene of World War One battles" , "River of northern France" , "French river known from World War 1" , "WW1 battle site" , "French river; WW1 battle" .)