Dull sound of impact (4)
I believe the answer is:
thud
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">dull sound of impact</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)