Sediment in the Nile estuary (4)
I believe the answer is:
lees
'sediment' is the definition.
(I know that lees is a type of sediment)
'in the nile estuary' is the wordplay.
'in' indicates the answer is hidden within the clue.
'LEES' is hidden within 'nile estuary'.
(Other definitions for lees that I've seen before include "Sediement or else" , "Eels found in sediment" , "Sediment that settles at the bottom of wine" , "Dregs, sediment" , "Wine dregs" .)