Riverbank deposit (4)
I believe the answer is:
silt
'riverbank deposit' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for silt that I've seen before include "Material deposited by a river" , "Mud deposits in a river" , "Slimy mud" , "Fine sediment (often deposited by water)" , "Sedimentary material" .)