Mattress support (4)
'mattress support' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for bedstead that I've seen before include "Framework supporting springs and mattresses" , "support for the retired" , "Frame upon which a mattress sits" , "Frame etc on which one sleeps" , "Frame for a sleeping person" .)
(Other definitions for slat that I've seen before include "Part of a Venetian blind" , "Thin strip" .)
(Another definition for slats that I've seen is " Thin strips of wood or metal".)