Delta deposit (4)
I believe the answer is:
'delta deposit' 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 "Sediment deposited by water as in channel" , "Material deposited by a river" , "Sediment deposited by a river" , "Jamboree" , "Mud deposit in a river" .)