how do I clear a stolen iPhone

From your computer, go to icloud.com/find. Click the phone you want to erase, then click Erase iPhone. If the phone is online, it will start erasing right away. If it's offline, it will start erasing the next time it connects to the internet.

Erase a device in Find Devices on iCloud.com - Apple Support

If you didn't have Find My enabled on your phone when you still had access to the device, you can't track or erase your phone.

If bumbieben0’s trick didnt work try resettting by holding the power and volume up and home button if it is buttonless click volumeup and volumedown and the powerbutton until it goes black