Henry has not gone back to bed (6)
I believe the answer is:
cotton
'henry' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'not gone back to bed' is the wordplay.
I cannot quite understand how this works, but
an anagram of 'not' is 'ton' which is within the answer.
'bed' could be 'cot' (cot is a type of bed) and 'cot' is present in the answer.
No letters remain.
This explanation may well be incorrect...
'has' is the link.
Can you help me to learn more?
(Other definitions for cotton that I've seen before include "Fabric woven from plant fibres" , "Source of cloth" , "Creamy-white natural fibre" , "Natural cloth" , "Kind of thread or fabric" .)