Musical nun holding note brings complaint (7)
I believe the answer is:
malaria
'complaint' is the definition.
(malaria is a kind of health problem)
'musical nun holding note' is the wordplay.
I cannot quite see how this works, but
'note' could be 'la' (musical note as in do-re-mi) and 'la' is found within the answer.
'musical' could be 'maria' (I've seen this before) and 'maria' is found within the remaining letters.
No letters remain.
This explanation may well be incorrect...
'brings' is the link.
Can you help me to learn more?
(Other definitions for malaria that I've seen before include "Illness" , "Disease spread by mosquito bites" , "Fever-inducing disease" , "Disease carried by mosquitos" , "Disease from bites by infected mosquitoes" .)