curtain-raiser

noun2

1. •
   In a theater, an initial play, musical performance, etc., which precedes the main performance.

   • [H]e had tried all kinds: comedy, tragedy, prose and verse, the light curtain-raiser, the short sharp drama, the bourgeoisrealistic and the lyrical-romantic. — 1910
2. •
   A prelude or introduction to an activity or thing.

   • It is my opinion that your part in the affair is only a curtain-raiser to graver things. — 1901