Alluvium
I believe the answer is:
silt
'alluvium' 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 "Jamboree" , "Slimy mud" , "Fine sediment (often deposited by water)" , "Fine deposited material" , "Sedimentary deposit" .)