A high note, discordant (3,3)
I believe the answer is:
off key
'discordant' is the definition.
(thesaurus)
'a high note' is the wordplay.
'a high' becomes 'off' (both can refer to rotten food).
'note' becomes 'key' (I can't explain this - if you can you should give a lot more credence to this answer).
'off'+'key'='OFF-KEY'
Can you help me to learn more?
(Other definitions for off key that I've seen before include "Inaccurate in pitch" , "Musically, not quite on the right note" , "using the wrong pitch" , "Musically, out of tune" , "played badly" .)