Without a key (6)
I believe the answer is:
atonal
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">without a key</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)