Sound of a heavy fall (4)
I believe the answer is:
thud
'sound of a heavy fall' 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 "Sound of a heavy fall" , "Clonk" , "Noise of dull impact" , "Dull bang" , "Dull heavy noise" .)