Sleep with back of neck not completely visible (3)
I believe the answer is:
nap
'sleep' is the definition.
('nap' can be a synonym of 'sleep')
'back of neck not completely visible' is the wordplay.
'back of neck' becomes 'nape' (I've seen this before).
'not completely visible' means to remove the last letter.
'nape' with its final letter removed is 'NAP'.
'with' acts as a link.
(Other definitions for nap that I've seen before include "Raised surface of a fabric and short sleep" , "Sleep lightly" , "raised bits of velvet" , "Racing tip that professes to be a certainty" , "A daytime doze" .)