Baltic Sea feeder (4)
I believe the answer is:
oder
'baltic sea feeder' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for oder that I've seen before include "River forming northern part of the German-Polish border" , "East European river" , "Rode around the European river" , "River on border of Poland and Germany" , "Polish/German river" .)