Like Berg's music (6)
I believe the answer is:
atonal
'like berg'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) without key" , "Keyless" , "(Of music) not in any key or mode" , "As modernist works" , "Lacking traditional musical tone structure" .)