Bed not turned - but the sheets may be (6)
I believe the answer is:
'the sheets may be' is the definition.
I can't tell whether this definition defines the answer.
'bed not turned' is the wordplay.
'bed' becomes 'cot' (cot is a kind of bed).
'turned' says the letters should be written in reverse.
'not' backwards is 'ton'.
'but' acts as a link.
Can you help me to learn more?
(Other definitions for cotton that I've seen before include "Substance like fine wool" , "Cloth woven from a plant" , "Golfing man" , "Soft fabric and fluffy wool (6" , "Fabric woven from plant fibres" .)