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

1. General's helper
2. Royal emblem of France
3. Final stroke
4. Stroke of genius
5. Gusto
6. Mark Twain, e.g.
7. National holiday of Mexico
8. Street with no outlet

Answers:

1. Aide de camp
2. Fleur de lis
3. Coup de grace
4. Tour de force
5. Joie de vivre
6. Nom de plume
7. Cinco de Mayo 
8. Cul de sac

Credit:
Katherine Omak
FIDDLE-DE-DEE