Troublesome problem or nasty pain (8)
I believe the answer is:
headache
'troublesome problem' is the definition.
'headache' can be an answer for 'problem' (headache can mean a problem or annoyance). I am not certain of the 'troublesome' bit.
'nasty pain' is the wordplay.
'nasty' becomes 'head' (I am not sure about this - if you are sure you should believe this answer much more).
'pain' becomes 'ache' (ache is a kind of pain).
'head'+'ache'='HEADACHE'
'or' is the link.
Can you help me to learn more?
(Other definitions for headache that I've seen before include "A pain in the brain" , "Cause of worry or trouble" , "Worry, trouble (colloq.)" , "Cause of worry/trouble" , "Problem, nuisance" .)