Quiet nobleman is a gem (5)
I believe the answer is:
pearl
'gem' is the definition.
(I know that pearl is a type of gem)
'quiet nobleman' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'nobleman' becomes 'earl' (member of the nobility).
'p'+'earl'='PEARL'
'is a' is the link.
(Other definitions for pearl that I've seen before include "Precious stone from shellfish" , "Smooth lustrous round structure of calcium carbonate" , "Gem from oyster" , "Much loved and valued person" , "Gem; an attacked Harbor" .)