River sediment (4)
I believe the answer is:
silt
'river sediment' 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 "Mud deposit in a river" , "Muddy deposit" , "Fine sediment (often deposited by water)" , "Fine river sediment" , "Material deposited by a river" .)