Material in bed not turned over (6)
I believe the answer is:
cotton
'material' is the definition.
(cotton is a kind of material)
'bed not turned over' is the wordplay.
'bed' becomes 'cot' (cot is a kind of bed).
'turned over' is a reversal indicator.
'not' reversed gives 'ton'.
'cot'+'ton'='COTTON'
'in' acts as a link.
(Other definitions for cotton that I've seen before include "Fabric (of natural fibre)" , "Fabric woven from plant fibres" , "Source of cloth" , "Plantation product" , "Substance like fine wool" .)