Dull heavy sound (4)
I believe the answer is:
thud
'dull heavy sound' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for thud that I've seen before include "Clonk" , "Dull, heavy sound" , "Dull heavy noise" , "Sound of a heavy fall" , "Dull sound made by a falling body" .)