The answers to the clues below will have something in common.

1. Illusion, as of a ghost
2. Fancy clothes, as for church
3. Arena-shaped venue for stage plays
4. Expression of derision directed at pretentious behavior
5. Hearing about someone/thing from someone else
6. Legal proceeding with a guaranteed successful result

Answers:

Each answer is an expression with three or more hyphenated words.

1. Will-o'-the wisp 
2. Sunday-go-to-meetin'
3. Theatre-in-the-round
4. La-di-da 
5. Word-of-mouth
6. Open-and-shut case

Credit: 
1-4: Merl Reagle
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