Bed covering a blanket (4)
I believe the answer is:
coat
'blanket' is the definition.
(I've seen this before)
'bed covering a' is the wordplay.
'bed' becomes 'cot' (cot is a kind of bed).
'covering' means one lot of letters goes inside another.
'cot' enclosing 'a' is 'COAT'.
(Other definitions for coat that I've seen before include "Warm outerwear" , "Jacket" , "Outer garment" , "Layer of paint" , "Outer covering" .)