It's bed-time - get on with the yarn (6)
I believe the answer is:
cotton
'the yarn' is the definition.
(cotton is a kind of yarn)
'it's bed-time get on' is the wordplay.
'it's bed' becomes 'cot' (cot is a kind of bed).
'time' becomes 't' (abbreviation).
'get' means one lot of letters go next to another.
'cot'+'t'+'on'='COTTON'
'with' acts as a link.
(Other definitions for cotton that I've seen before include "Golfing man" , "Soft fabric and fluffy wool (6" , "Fabric made from tropical plant - old man river don't plant it" , "Plant fibre for fabrics" , "Fabric woven from plant fibres" .)