Keyless (6)
I believe the answer is:
atonal
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">keyless</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for atonal that I've seen before include "Keyless" , "(Music) having no established key" , "Unaccented" , "(Of music) without key" , "(Of music) not in any key or mode" .)