Point duet out – a piece of music (5)
I believe the answer is:
etude
'a piece of music' is the definition.
(I know this)
'point duet out' is the wordplay.
'point' becomes 'e' (compass point).
'out' is an anagram indicator (out can mean wrong or inaccurate).
'duet' anagrammed gives 'tude'.
'e'+'tude'='ETUDE'
(Other definitions for etude that I've seen before include "bit of practice" , "short musical piece" , "Testing musical composition" , "Short composition for a solo instrument" , "A short musical exercise" .)