River to the Baltic (4)
I believe the answer is:
oder
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">river to the baltic</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for oder that I've seen before include "River flowing into the Baltic" , "waterway" , "flowing water" , "river, one in central Europe" , "European flower" .)