Music not written in a particular key (6)
I believe the answer is:
atonal
'music not written in a particular key' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for atonal that I've seen before include "(Music) having no established key" , "As modernist works" , "Describes music not written in a particular key" , "(Of music) without key" , "Unaccented" .)