Series of eight notes (6)
I believe the answer is:
octave
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">series of eight notes</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 octave that I've seen before include "Interval involving a doubling of frequency" , "Series of eight consecutive musical notes" , "Musical range" , "eight days starting with a festival" , "A group of eight consecutive musical notes" .)