She's soft on an aristocrat (5)
I believe the answer is:
pearl
'she's' is the definition.
'soft on an aristocrat' is the wordplay.
'soft' becomes 'p' (musical abbreviation for 'piano').
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'an aristocrat' becomes 'earl' (earl is a kind of aristocrat).
'p'+'earl'='PEARL'
(Other definitions for pearl that I've seen before include "Lustrous globule prized as a gem" , "White - jewel" , "one regularly in stitches" , "Girl < , pretty" , "Precious stone from shellfish" .)