A series of eight musical notes (6)
I believe the answer is:
octave
'a series of eight musical notes' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for octave that I've seen before include "Interval involving a doubling of frequency" , "double the frequency?" , "eight days starting with a festival" , "A verse of eight lines" , "Interval of 12 semitones" .)