Bed has a layer (4)
I believe the answer is:
coat
'layer' is the definition.
(I have seen 'Layer of paint' mean 'coat' so perhaps 'layer' could also mean 'coat')
'bed has a' is the wordplay.
'bed' becomes 'cot' (cot is a type of bed).
'has' means one lot of letters goes inside another.
'cot' enclosing 'a' is 'COAT'.
(Other definitions for coat that I've seen before include "blanket" , "Warm outerwear" , "Outer garment" , "Overgarment" , "Fleece" .)