Land heavily (4)
I believe the answer is:
plop
'land heavily' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for plop that I've seen before include "Sound of object falling into water without a splash" , "Noise of a small object dropping into water" , "Sound of a stone hitting water" , "Report making little splash" , "Sound of a stone landing in water" .)