Quiet nobleman is gem (5)
I believe the answer is:
pearl
'is gem' is the definition.
(I've seen this before)
'quiet nobleman' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'nobleman' becomes 'earl' (earl is a kind of nobleman).
'p'+'earl'='PEARL'
(Other definitions for pearl that I've seen before include "Lustrous structure inside an oyster-shell" , "Gem; an attacked Harbor" , "greatly valued" , "Girl < , pretty" , "Smooth lustrous round structure of calcium carbonate" .)