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