She's soft on a nobleman (5)
I believe the answer is:
pearl
'she's' is the definition.
'soft on a nobleman' is the wordplay.
'soft' becomes 'p' (musical abbreviation for 'piano').
'on' is a charade indicator (letters next to each other).
'a nobleman' becomes 'earl' (earl is a kind of nobleman).
'p'+'earl'='PEARL'
(Other definitions for pearl that I've seen before include "See 12" , "Precious gem" , "Object of great worth" , "Lustrous globule prized as a gem" , "Precious stone from shellfish" .)