Creating a keyboard shortcut for Numbers

I don't know with the latest OS but here are instructions for making app shortcuts in Mac OS 12.7.3:

1. System Preferences->Keyboard->Shortcuts
2. Choose App Shortcuts from the list on the left
3. Click the + to add a shortcut
4. Select the application (Numbers)
5. Type the menu title exactly as it appears in the menu. Upper/lower case matters.
6. Click in the Shortcut box then press the combination of keys you want
7. Click on Add

I am of the impression that things have changed but maybe this will get you started.

If you mean Edit > Clear all you can also just hit the <delete> key (after selecting the cells).

SG