Without a key (6)
I believe the answer is:
atonal
'without a key' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for atonal that I've seen before include "(Of music) without key" , "As modernist works" , "Keyless" , "Musically, having no established key" , "An alto (anag) - without a fixed key" .)