River mouth feature (5)
I believe the answer is:
delta
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">river mouth feature</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for delta that I've seen before include "Fragmented estuary" , "Divided river outlet" , "Type of wing" , "Area of distributaries at a river mouth" , "Wing shape; Greek D" .)