Rachmaninoff's instrument (5)
I believe the answer is:
piano
'rachmaninoff's instrument' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for piano that I've seen before include "Soft, softly (music)" , "quiet passage" , "Softly, musically speaking" , "In musical notation, this means softly" , "one of 6 [STEINWAYS]" .)