Poor-sounding judge makes a mark (9)
I believe the answer is:
diacritic
'a mark' is the definition.
(diacritic is a kind of mark)
'poor-sounding judge' is the wordplay.
'poor-sounding' becomes 'dia' (I can't justify this - if you can you should give a lot more credence to this answer).
'judge' becomes 'critic' (critic is a kind of judge).
'dia'+'critic'='DIACRITIC'
'makes' acts as a link.
Can you help me to learn more?
(Other definitions for diacritic that I've seen before include "accent, for example" , "Mark" , "Capable of distinguishing" , "grave perhaps" , "[ACCENT] , perhaps" .)