Water balloon sound (5)
I believe the answer is:
splat
'water balloon sound' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for splat that I've seen before include "Noise of soft wet impact; part of a chair back" , "amusing noise" , "Sound of a soft wet landing" , "Noise on landing" , "Wet slapping sound" .)