Very deep sleep (4)
I believe the answer is:
coma
'very deep sleep' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for coma that I've seen before include "Deep sleep" , "State of insensibility" , "Unconsciousness from which person cannot be aroused" , "Long period of unconsciousness" , "long sleep" .)