Moselle tributary (4)
I believe the answer is:
saar
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">moselle tributary</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 saar that I've seen before include "German wine valley" , "River of France and Germany" .)