You can use adb to do this without root (But you need a PC). First download Android platform tools from https://developer.android.com/studio/releases/platform-tools now unzip them in a folder in your PC.
Go to that folder and open Command Prompt. Now in your phone, find the build number and tap it 7-8 times to enable Developer Options. Now open Developer Options and enable USB debugging. Then plug your phone with your PC. Confirm the box.
Now first type adb devices
. Then enter the command adb shell
to start adb shell. Now type the command pm list packages | grep "clock"
or if you have problem with grep then use pm list packages | find "clock"
. Find the package name of default clock. Now use pm uninstall -k --user 0 package-name
. If you see Success message, then clock app will be removed.