Overact manically: spectator finally bailing in interval (6)
I believe the answer is:
octave
'interval' is the definition.
(musical interval)
I don't understand how the remainder of the clue works.
(Other definitions for octave that I've seen before include "Twelve semitones" , "At cove (anag.)" , "Series of eight notes in music" , "eight days starting with a festival" , "double the frequency?" .)