Slumber (5)
I believe the answer is:
sleep
'slumber' is the definition.
(I know that slumber can be written as sleep)
This is all the clue.
(Other definitions for sleep that I've seen before include "crash" , "''... that knits up the ravell'd sleeve of care'' (Macbeth)" , "Doze, nap" , "Natural state of unconscious rest" , "Be dormant" .)