I understand that you want to delete your game progress for Clash Royale on your iPhone. I know that it's important to have control over your data, so I'm happy to help.
I've found this on the Clash Royale support site:
Supercell Support
How do I reset my account?
The fun of Clash Royale is sticking with your account through the good times and the bad times, so the game wasn't designed for being reset and restarted at a whim.
You can only start a new game on a new device, but remember that your original game stays connected to your Game Center account, and this can't be switched.
If you find that you need to sign into a different Game Center account, you can find out more about that here:
Sign in with a different Apple ID on your iPhone, iPad, or iPod touch - Apple Support
Game Center
- Tap Settings > Game Center. If you see the option to sign in, you can change your Apple ID now.
- Tap your Apple ID, then tap Sign Out.
- Tap Sign In, then enter your Apple ID and password.
Thanks for using the Apple Support Communities. Cheers!