Cause of worry or trouble (8)
I believe the answer is:
headache
I believe this is a double definition.
'cause of worry' is the first definition.
(I've seen this before)
'trouble' is the second definition.
(thesaurus)
(Other definitions for headache that I've seen before include "Problem, nuisance" , "Cause of worry or trouble" , "A pain in the brain" , "Worry, trouble (colloq.)" , "Pain in the cranium" .)