Piece of music for piano (6)
I believe the answer is:
sonata
'piece of music for piano' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for sonata that I've seen before include "Composition for 1 or 2 instruments, with 3 or 4 movements" , "musical performance" , "composition for piano, perhaps" , "Classical composition" , "Beethoven's 'Moonlight', for example" .)