River mouth formation (5)
I believe the answer is:
delta
'river mouth formation' 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 "Triangular patch of land at mouth of river" , "Fourth Greek letter? - area at river mouth" , "Greek letter in the shape of a triangle" , "Nile river mouth" , "Silted estuary" .)