Thin mud (4)
(Other definitions for silt that I've seen before include "Slimy mud" , "Mud deposit in a river" , "Mud deposited in a river or lake" , "Sediment deposited by a river" , "Fine sediment (often deposited by water)" .)
'thin mud' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for slime that I've seen before include "Goo" , "Slippery mud; slug trail" , "Gunge" , "Nasty slippery stuff" , "Moist slippery stuff" .)