He may not have washed the back of his neck (6)
I believe the answer is:
scruff
This could be a double definition.
'he may not have washed' is the first definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'back of his neck' is the second definition.
The answer and definition can be both body parts as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'the' is the link.
Can you help me to learn more?
(Other definitions for scruff that I've seen before include "One probably wouldn't wash" , "bit of neck?" , "Untidy person - nape of neck" , "Nape; untidy type" , "Untidy person (slang); nape" .)