Wet slapping sound (5)
I believe the answer is:
splat
'wet slapping sound' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for splat that I've seen before include "Piece of wood in a chair back" , "Sound of a wet and squishy impact" , "Wet slapping sound" , "Noise on landing" , "Noise of soft wet impact; part of a chair back" .)