Riverbed sediment (4)
I believe the answer is:
silt
'riverbed sediment' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for silt that I've seen before include "Mud deposit in a river" , "Fine river sediment" , "Sediment in a stream" , "Fine deposited material" , "Sediment deposited in channel or harbour" .)