Duet changed key to another piece (5)
I believe the answer is:
etude
'another piece' is the definition.
'etude' can be an answer for 'piece' (etude is a kind of piece). I'm not certain of the 'another' bit.
'duet changed key' is the wordplay.
'changed' indicates an anagram.
'key' becomes 'e' (a musical key).
'duet' is an anagram of 'etud'.
'etud'+'e'='ETUDE'
'to' is the link.
Can you help me to learn more?
(Other definitions for etude that I've seen before include "bit of practice" , "Chopin study" , "short musical piece" , "Testing musical composition" , "Short piece of music for solo instrument" .)