Medical problem? Good, everything's sound (9)
I believe the answer is:
gallstone
'medical problem?' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'good everything's sound' is the wordplay.
'good' becomes 'g' (abbreviation).
'everything's' becomes 'all's' (all things).
'sound' becomes 'tone' (tone is a kind of sound).
'g'+'alls'+'tone'='GALLSTONE'
Can you help me to learn more?
(Other definitions for gallstone that I've seen before include "It's removed in hospital" , "Concretion of cholesterol, lime salts, etc" , "Let Anglos suffer with small hard mass in body" , "Small hard secretion formed in the body" , "Problem for bile ducts" .)