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' (I've seen this in other clues).
'note' becomes 'key' (I am not sure about this - if you are sure 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" .)