Polish river, or German (4)
I believe the answer is:
oder
This clue is a double definition.
'polish river' is the first definition.
(River Oder in central Europe)
'or german' is the second definition.
('or' in German)
(Other definitions for oder that I've seen before include "flowing water" , "River forming northern part of the German-Polish border" , "European flower" , "East European river" , "River on border of Poland and Germany" .)