Sediment looks full of unpleasantness after river drops (4)
I believe the answer is:
lees
'sediment' is the definition.
(I know that lees is a type of sediment)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for lees that I've seen before include "Sediment that settles at the bottom of liquid" , "Sediment from fermentation of alcohol" , "Dregs of drink" , "Dregs left by eels" , "Sediment from wine or beer production" .)