Cloth, not new, behind bed (6)
I believe the answer is:
cotton
'cloth' is the definition.
(cotton is a kind of cloth)
'not new behind bed' is the wordplay.
'new' indicates an anagram.
'behind' means one lot of letters go next to another.
'bed' becomes 'cot' (cot is a kind of bed).
'not' with letters rearranged gives 'ton'.
'ton' after 'cot' is 'COTTON'.
(Other definitions for cotton that I've seen before include "Fabric (of natural fibre)" , "Substance like fine wool" , "Cloth woven from a plant" , "Kind of thread or fabric" , "Plant fibre for fabrics" .)