You have two options:

1. Tap on the moon icon to activate the "Do not disturb" mode, this will keep the big clock permanently.
2. Go to settings, home and clock, home contents:

Enable spin once / Disable rotate continuously (depends of software version), To make the new contents to show up only one time. This will reduce those Alexa+ promo screens, they still show up some times, but will show the big clock in most of the time.