Duck lower for cover (9)
I believe the answer is:
eiderdown
'cover' is the definition.
(thesaurus)
'duck lower' is the wordplay.
'duck' becomes 'eider' (eider is a kind of duck).
'lower' becomes 'down' ('down' can be a synonym of 'lower').
'eider'+'down'='EIDERDOWN'
'for' acts as a link.
(Other definitions for eiderdown that I've seen before include "What might keep one warm" , "Warm bed cover" , "what ducks have to offer us" , "cover when retired" , "Soft quilt filled with duck feathers" .)