Sat on a different piece (6)
I believe the answer is:
sonata
'piece' is the definition.
(thesaurus)
'sat on a different' is the wordplay.
'different' indicates anagramming the letters.
'sat'+'on'+'a'='satona'
'satona' anagrammed gives 'SONATA'.
(Other definitions for sonata that I've seen before include "Solo instrumental composition" , "To NASA, strangely, for the solo piece" , "Sat on a kind of composition" , "musical performance" , "composition for piano, perhaps" .)