Quiet set of keys (5)
I believe the answer is:
piano
I believe this clue is a double definition.
'quiet' is the first definition.
(I've seen this before)
'set of keys' is the second definition.
(I've seen this in another clue)
(Other definitions for piano that I've seen before include "soft passage" , "Upright type?" , "Keyboard instrument" , "key holder" , "Musical instrument - softly, please" .)