Fall sound (4)
I believe the answer is:
thud
'fall sound' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for thud that I've seen before include "Dull heavy impact" , "Noise of dull impact" , "Clonk" , "Dull, heavy sound" , "Sound made by impact of heavy objects" .)