how do i restore the built in clock icon on home screen that i accidentally deleted

Go To Settings

Select General

Select iPhone Storage

Scroll down until you find the delete app

Select the deleted app

The screen will give you two options - Offload App or Delete App, select Delete App

Go back to app store and download the app

With iOS 14 apps can be removed from the home screen without being deleted. If you have iOS 14, go to the App Library (swipe left past your last home screen). From there you can touch and hold an app and slide it left back to the home screen.

If that is not the cause, try using Search to see if that will locate it: https://support.apple.com/en-us/HT201285

If it was deleted, see the following from Delete built-in Apple apps from the Home screen on your iOS 14 or iPadOS device or Apple Watch - Apple Support

Restore a built-in app that you deleted

1. On your iOS or iPadOS device, go to the App Store.
2. Search for the app. Make sure that you use the exact name of the app. Find the correct name of built-in apps.
3. Tap  to restore the app.
4. Wait for the app to restore, then open it from your Home screen.