Historical clue formulations

• Classic work by 50- and 23-Down, so to speak? — 2018-04-11
• Gentle air — 1999-09-11
• Introspective composition — 1999-01-02
• Chopin specialty — 1998-06-10
• Feature of Mendelssohn's "A Midsummer Night's Dream" — 1998-03-04