Duck on the German river (4)
I believe the answer is:
oder
'river' is the definition.
(Oder River is an example)
'duck on the german' is the wordplay.
'duck' becomes 'o'.
'on' is a charade indicator (letters next to each other).
'the german' becomes 'der' ('the' in German**).
'o'+'der'='ODER'
(Other definitions for oder that I've seen before include "German and Polish river" , "Polish/German river" , "East European river" , "European flower" , "River that, with the Neisse, forms the German-Polish border" .)