Habitual sleeplessness (8)
I believe the answer is:
insomnia
'habitual sleeplessness' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for insomnia that I've seen before include "Not getting off" , "Inability to drop off" , "Condition affecting sleep" , "problem at night" , "State of chronic sleeplessness" .)