Quiet instrument (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)
'instrument' is the second definition.
(musical instrument)
(Other definitions for piano that I've seen before include "Softly, musically speaking" , "In musical notation, this means softly" , "Soft, softly (music)" , "Franz Liszt's instrument" , "Soft - musical instrument" .)