River mouth deposit (5)
I believe the answer is:
delta
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">river mouth deposit</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 delta that I've seen before include "Daedalus started with this" , "Wing shape; Greek D" , "Wide silted river mouth" , "Area of distributaries in a river basin" , "Area where a river divides before entering the sea" .)