He seems not to have washed the back of the neck (6)

I believe the answer is:

scruff

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'back of the neck' is the definition.
(I've seen this before)

I can't explain the rest of the clue.

(Other definitions for scruff that I've seen before include "Unkempt individual" , "One probably wouldn't wash" , "One hardly washing" , "Back of the neck, used to lift cat" , "Back of the neck; untidy person" .)

I've seen this clue in The Globe and Mail.

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