Duck (down under it as cover) (5)
I believe the answer is:
eider
I believe this is a double definition.
'duck' is the first definition.
(eider is a type of duck)
'down under it as cover' is the second definition.
(as in eiderdown bedcovers)
(Other definitions for eider that I've seen before include "Duck valued for its fine soft down" , "Down comes from this" , "Sea bird" , "Species of duck" , "Large sea duck" .)