Monsieur is immersed in unspecified French river (5)
I believe the answer is:
somme
'french river' is the definition.
(the Somme is a river in France)
'monsieur is immersed in unspecified' is the wordplay.
'monsieur' becomes 'm' (French abbreviation - like Mr for Mister).
'is immersed in' means one lot of letters goes inside another.
'unspecified' becomes 'some' (similar in meaning).
'm' inserted inside 'some' is 'SOMME'.
(Other definitions for somme that I've seen before include "French 2 down" , "Scene of WW One battles" , "World War One battle region" , "World War one battle where great losses were incurred" , "French river known from World War 1" .)