Composition from 'Peter Grimes', on at Aldeburgh (6)
I believe the answer is:
sonata
'composition' is the definition.
(thesaurus)
'from peter grimes on at aldeburgh' is the wordplay.
'from' indicates the answer is hidden within the clue.
'SONATA' can be found hidden inside 'peter grimes on at aldeburgh'.
(Other definitions for sonata that I've seen before include "just for one or two players?" , "Composition for 1 or 2 instruments, with 3 or 4 movements" , "A musical form" , "Piece of classical music" , "Beethoven's 'Moonlight', for example" .)