Cephalalgia can be a nuisance (8)
I believe the answer is:
headache
I believe this clue is a double definition.
'cephalalgia' is the first definition.
(I know that cephalalgia can be written as headache)
'a nuisance' is the second definition.
(thesaurus)
'can be' is the link.
(Other definitions for headache that I've seen before include "big problem" , "Problem, nuisance" , "Pain in the cranium" , "Source of worry (and pain!)" , "trial" .)