Pie-in-the-face sound (5)
I believe the answer is:
splat
'pie-in-the-face 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 "Sound of a wet and squishy impact" , "Noise on landing" , "Wet slapping sound" , "Piece of wood in a chair back" , "Sound of a soft wet landing" .)