Treble’s final duet ruined piece (5)
I believe the answer is:
etude
'piece' is the definition.
(etude is a kind of piece)
'treble's final duet ruined' is the wordplay.
'final' suggests the final letters.
'ruined' indicates anagramming the letters.
The final letter of 'treble' is 'e'.
'duet' anagrammed gives 'tude'.
'e'+'tude'='ETUDE'
(Other definitions for etude that I've seen before include "Chopin study" , "Testing musical composition" , "Noted composition" , "Indian statesman" , "short musical piece" .)