Problem of pain (8)
I believe the answer is:
headache
I believe this clue is a double definition.
'problem' is the first definition.
(headache can mean a problem or annoyance)
'pain' is the second definition.
(headache is a kind of pain)
'of' is the link.
(Other definitions for headache that I've seen before include "Problem, nuisance" , "Informally a worrying problem - take an asprin" , "Cause of worry/trouble" , "trial" , "nasty problem" .)