Disease’s evidence of harm felt wrong always (7,5)
I believe the answer is:
scarlet fever
'disease's' is the definition.
(scarlet fever is a kind of health problem)
'evidence of harm felt wrong always' is the wordplay.
'evidence of harm' becomes 'scar' (I can't explain this - if you can you should believe this answer much more).
'wrong' is an anagram indicator.
'always' becomes 'ever' (synonyms).
'felt' with letters rearranged gives 'letf'.
'scar'+'letf'+'ever'='SCARLET FEVER'
Can you help me to learn more?
(Other definitions for scarlet fever that I've seen before include "Infectious disease producing rash" , "complaint" , "Disease causing rash" , "Infectious disease of children" .)