Ruddy duck back in river (4)
I believe the answer is:
oder
'river' is the definition.
(Oder River is an example)
'ruddy duck back' is the wordplay.
'ruddy' becomes 'red' ('red' can be a synonym of 'ruddy').
'duck' becomes 'o'.
'back' is a reversal indicator.
'red'+'o'='redo'
'redo' back-to-front is 'ODER'.
'in' acts as a link.
(Other definitions for oder that I've seen before include "River on border of Poland and Germany" , "Rode around the European river" , "River flowing into the Baltic" , "River that, with the Neisse, forms the German-Polish border" , "European banker" .)