"Clunk!" (4)
I believe the answer is:
thud
'clunk' is the definition.
(I know that clunk can be written as thud)
This is the entire clue.
(Other definitions for thud that I've seen before include "Dull sound made by a falling body" , "Dull, heavy sound" , "Noise of dull impact" , "Dull heavy impact" , "Clonk" .)