Brook, small stream (4)
I believe the answer is:
rill
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">brook</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >'<span class="explanation-format__original">small stream</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(geographical term)</span><br/ ><br/ ></span>
(Other definitions for rill that I've seen before include "Flowing water" , "Burn" , "Small stream" , "Brooklet" , "Small brook" .)