Extract from commentator on doubtful piece of music (5)
I believe the answer is:
rondo
'piece of music' is the definition.
(type of musical composition)
'extract from commentator on doubtful' is the wordplay.
'extract from' indicates the answer is hidden within the clue (some letters extracted from the clue).
'RONDO' is hidden in the letters of 'commentator on doubtful'.
(Other definitions for rondo that I've seen before include "Musical form with repeated theme" , "rota, maybe" , "Piece of music with recurring theme" , "Repetitive movement" , "Last movement of a sonata or concerto" .)