Bang (4)
(Other definitions for thud that I've seen before include "Dull heavy noise" , "Dull, heavy sound" , "Sound of a heavy fall" , "Sound made by impact of heavy objects" , "Dull bang" .)
'bang' is the definition.
(I know that bang can be written as slam)
This is all the clue.
(Other definitions for slam that I've seen before include "Hit hard" , "ambitious bid" , "Taking of twelve or thirteen tricks (bridge)" , "Severely criticise; shut (a door) forcefully" , "major contract" .)