Dull sound of impact (4)
I believe the answer is:
thud
'dull sound of impact' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for thud that I've seen before include "Sound of heavy blow" , "Low dull sound as of blow" , "Dull heavy noise" , "Dull, heavy sound" , "Dull bang" .)