As a side note, notice Clarabelle's name printed above her door. This is the first time we the audience have gotten a name for the character.
Clarabelle is seen reading in her hay loft when Horace arrives. One thing I love about these cartoons is the fact that they are time capsules of a bygone period of time. Clarabelle's book, Three Weeks, was banned in 1907 for distasteful reasons. It's unclear why she's seen here reading it.
Once Clarabelle hears Horace at her door, she quickly dresses and is ready for a night out. Since Clarabelle is technically naked, this scene has been censored by Disney when aired on TV.
The Shindig is full of lively tunes and great animation of animals dancing. While Mickey and Minnie play on their fiddle and piano, Horace and Clarabelle become the center of attention on the dance floor. This is yet another way the two characters were beginning to be pushed into the spotlight alongside Mickey and Minnie.