An alto can be discordant (6)
I believe the answer is:
atonal
'discordant' is the definition.
(thesaurus)
'an alto can be' is the wordplay.
'can be' indicates anagramming the letters.
'an'+'alto'='analto'
'analto' anagrammed gives 'ATONAL'.
(Other definitions for atonal that I've seen before include "Describes music not written in a particular key" , "Keyless" , "Lacking traditional musical tone structure" , "(Of music) without key" , "Unaccented" .)