Piece of wood in a chair back (5)
I believe the answer is:
splat
'piece of wood in a chair back' is the definition.
(I've seen this before)
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" , "Wet slapping sound" , "amusing noise" , "Sound of a soft, wet object striking a surface" , "Sound of a soft wet landing" .)