Thus madame is seen in one of her rivers? (5)
I believe the answer is:
somme
'seen in one of her rivers?' is the definition.
I can't judge whether this defines the answer.
'thus madame' is the wordplay.
'thus' becomes 'so' (synonyms).
'madame' becomes 'mme' (abbreviation).
'so'+'mme'='SOMME'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for somme that I've seen before include "1916 battleground" , "Scene of WW One battles" , "World War One battle region" , "First World War battlefield" , "Scene of World War One battles" .)