Possible Clues:

• C neighbor
• Key of Prokofiev's Piano Concerto No. 1
• C sharp
• Key of Chopin's "Raindrop Prelude"
• ''Clair de lune'' key
• Key of ''Clair de Lune''
• Major key that uses all five black keys on a piano
•     C sharp equivalent
• Liszt's 'Consolation No. 3' is in it
• C sharp's soundalike