River mouth deposit (5)
I believe the answer is:
delta
'river mouth deposit' is the definition.
(I've seen this in another clue)
This is all the clue.
(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" .)