Some of this river encircles central Germany, but it's in France (5)
I believe the answer is:
somme
This might be a double definition.
'some of this river encircles central germany' is the first definition.
'somme' can be an answer for 'river' (I've seen this before). I'm unsure of the rest of the definition.
'it's in france' is the second definition.
('Somme' is part of 'france')
'but' is the link.
Can you help me to learn more?
(Other definitions for somme that I've seen before include "Battle site in France" , "French river at Amiens" , "Major Great War battle" , "bloody fight" , "Scene of WW One battles" .)