Not all monsieur caught in the river (5)
I believe the answer is:
somme
'river' is the definition.
(I've seen this before)
'not all monsieur caught in' is the wordplay.
'not all' becomes 'some' ('some' is the opposite to 'all').
'monsieur' becomes 'm'.
'caught in' indicates putting letters inside.
'some' enclosing 'm' is 'SOMME'.
'the' acts as a link.
(Other definitions for somme that I've seen before include "World War One battle region" , "WW1 battlefield" , "European river" , "bloody fight" , "French river at Amiens" .)