Onset unnoticed in non-specific health problem (4)
I believe the answer is:
ague
'health problem' is the definition.
(ague is a kind of health problem)
'onset unnoticed in non-specific' is the wordplay.
'onset' suggests taking the first letters (I've seen 'onset of' mean this).
'in' indicates putting letters inside.
'non specific' becomes 'age' (I can't explain this - if you can you should believe this answer much more).
The initial letter of 'unnoticed' is 'u'.
'u' placed into 'age' is 'AGUE'.
Can you help me to learn more?
(Other definitions for ague that I've seen before include "Feverish fit" , "Illness with fever and shivering, malaria say" , "Chills and fever" , "attack" , "Fever (archaic)" .)