Duck below the quilt (9)
I believe the answer is:
eiderdown
'the quilt' is the definition.
(eiderdown is a kind of quilt)
'duck below' is the wordplay.
'duck' becomes 'eider duck' (eider is a type of duck).
'below' becomes 'down' (in a down clue, some letters go below others).
'eider'+'down'='EIDERDOWN'
(Other definitions for eiderdown that I've seen before include "what keeps retired people warm" , "thermal insulation" , "I wondered about this warm bed covering" , "Warm bedcover" , "Find cover" .)