Sleeplessness (8)
I believe the answer is:
insomnia
'sleeplessness' is the definition.
(insomnia is a kind of sleeplessness)
This is all the clue.
(Other definitions for insomnia that I've seen before include "Wakefulness" , "In restless state" , "Sleep disorder" , "Habitual inability to sleep (8)" , "Habitual sleeplessness" .)