Piece of Mozart's on at Aldeburgh (6)
I believe the answer is:
sonata
'piece' is the definition.
(I've seen this before)
'of mozart's on at aldeburgh' is the wordplay.
'of' indicates a hidden word.
'SONATA' is hidden in the letters of 'mozarts on at aldeburgh'.
(Other definitions for sonata that I've seen before include "Music to play (not sing)" , "Instrumental composition for soloist" , "Musical piece" , "Music for a soloist" , "Chamber work" .)