Monsieur bathing in a little French river (5)
I believe the answer is:
somme
'french river' is the definition.
(I've seen this before)
'monsieur bathing in a little' is the wordplay.
'monsieur bathing' becomes 'm' (I am not sure about this - if you are sure you should believe this answer much more).
'in' indicates putting letters inside.
'a little' becomes 'some' (I've seen this before).
'm' inserted inside 'some' is 'SOMME'.
Can you help me to learn more?
(Other definitions for somme that I've seen before include "WW1 battle site" , "Battle site in France" , "Major Great War battle" , "1916 battleground" , "Scene of World War One battles" .)