Severe headache (8)
I believe the answer is:
migraine
'severe headache' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for migraine that I've seen before include "misery in capital area" , "Recurrent throbbing headache" , "Severe recurrent headache" , "Severe headache often accompanied by nausea" , "Long nerve fibre" .)