Series of eight notes (6)
I believe the answer is:
octave
'series of eight notes' is the definition.
(I've seen this before)
This is the entire clue.
(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" .)