Like Schoenberg's music (6)
I believe the answer is:
'like schoenberg's music' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for atonal that I've seen before include "(Of music) lacking key" , "(Of music) without key" , "Musically, having no established key" , "Not in a key (mus.)" , "Unaccented" .)