Sick headache (8)
I believe the answer is:
migraine
'sick headache' is the definition.
(I know that sick headache can be written as migraine)
This is all the clue.
(Other definitions for migraine that I've seen before include "Severe recurrent headache" , "misery in capital area" , "Thumping headache" , "It brings severe headache" , "A really nasty headache" .)