Only an eighth of a musical interval? (6)
I believe the answer is:
octave
This might be a double definition.
'only an eighth' is the first definition.
'octave' can be an answer for 'eighth' (thesaurus). I'm not certain of the 'only an' bit.
'a musical interval?' is the second definition.
(octave is a kind of musical interval)
'of' is the link.
Can you help me to learn more?
(Other definitions for octave that I've seen before include "Eight consecutive notes on music scale" , "Eight-note range" , "Musical interval doubling note frequency" , "Interval involving a doubling of frequency" , "Musical interval of eight tones" .)