I am successfully using 'defaults write com.apple.menuextra.clock IsAnalog -bool true' to replace to a smaller icon the Apple clock in Big Sur.
Is there a code to revert to Apple's clock if ever required?
MacOS – Menu Bar- clock-Big Sur
clockmacosmenu bar
Related Solutions
See https://github.com/rtrouton/create_macos_vm_install_dmg for a way to create a Big Sur installer disk image. For this to work, you need to download the Installer app onto your main Mac, run the script to create the disk image and then run Parallels with that image to create a Big Sur VM.
Performance may be rather unsatisfactory though, you might be better off with installing Big Sur on an external drive or separate partition, and boot from there.
In terminal, write:
defaults write com.apple.menuextra.clock IsAnalog -bool true
Not what you want but close enough as it makes it a single clock icon
Best Answer
Deleting a key will set it back to its default value.
However, you don't even need to delve into defaults to make this change, to or from analog — it's in the UI: System Preferences → Dock & Menu Bar → Clock → Time Options.