The stuff is not to be taken up to bed (6)
I believe the answer is:
cotton
'the stuff' is the definition.
(cotton is a kind of stuff)
'not to be taken up to bed' is the wordplay.
'to be taken up' says the letters should be written backwards.
'to' is a charade indicator (letters next to each other) (I've seen this in other clues).
'bed' becomes 'cot' (cot is a kind of bed).
'not' reversed gives 'ton'.
'ton' after 'cot' is 'COTTON'.
'is' acts as a link.
(Other definitions for cotton that I've seen before include "Soft white substance used to make fabric - in song, Old Man River don't plant it" , "Natural cloth" , "Cloth made from fine hairs of seed pod" , "Plant fibre for fabrics" , "Material" .)