The eighth musical interval? (6)
I believe the answer is:
octave
I believe this is a double definition.
'the eighth' is the first definition.
(thesaurus)
'musical interval?' is the second definition.
(octave is a kind of musical interval)
(Other definitions for octave that I've seen before include "Musical term - set of eight" , "Interval from one C to the next C" , "Musical interval of eight notes" , "Group of eight lines" , "Note an eighth above or below another" .)