Small narrow river (6)
I believe the answer is:
stream
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">small narrow river</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for stream that I've seen before include "class division" , "Small narrow flow" , "Flow of water on land" , "'Current, flow (6)'" , "Master of a tributary" .)