Deep sleep (4)
'deep sleep' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for coma that I've seen before include "long sleep" , "State of unconsciousness caused by illness" , "Stupor" , "Long period of unconsciousness" , "State of insensibility" .)
(Other definitions for slumber that I've seen before include "Drop off" , "Doze" , "Rumbles (anag.)" , "snores, maybe?" , "The rest" .)