Series of eight musical notes (6)
I believe the answer is:
octave
'series of eight musical notes' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for octave that I've seen before include "double the frequency?" , "eight days starting with a festival" , "Interval from one C to the next C" , "At cove (anag.)" , "A group of eight consecutive musical notes" .)