An eighth of the music (6)
I believe the answer is:
octave
'an eighth' is the definition.
(thesaurus)
'of the music' is the wordplay.
I cannot really understand how this works, but
'of' could be 'o' and 'o' is present in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is present in the answer.
'music' could be 'e' and 'e' is found within the answer.
The remaining letters 'cav' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for octave that I've seen before include "Musical interval of eight tones" , "Musical term - set of eight" , "Musical interval of eight notes" , "Series of eight notes on a scale" , "Eight consecutive notes on music scale" .)