Czech-Polish river (4)
I believe the answer is:
oder
'czech-polish river' 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 "German river that empties into the Baltic Sea" , "flowing water" , "River that, with the Neisse, forms the German-Polish border" , "River forming northern part of the German-Polish border" , "River flowing into the Baltic" .)