Possible Clues:

• Constable's superior, we hear, as artist
• Conductor was a right gracious fellow appearing at end of Proms
• Portraitist John Singer
• Not, you say, a top rank artist
• ''Madame X'' painter
• Painter superior to Constable, it's said
• Said he didn't rank highly as an artist
• American painter makes upper part of sky silver
• Noted 19th- and 20th-century portraitist
• In need of change, strange for a conductor