A piano piece? (3)
I believe the answer is:
key
'a piano piece?' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for key that I've seen before include "Lock undoer" , "Of crucial importance" , "Small island" , "//www.crosswordgiant.com/crossword-clue/1252911/" , "Crucially important" .)