My Aristo Android 7.0 does not have a clock widget
With a brief search I saw that there are lots and lots of clock widgets in google playstore. Install one from there and then go to the widgets list and at least one will be available.

examples:
https://play.google.com/store/apps/details?id=com.egert.clock&amp;hl=en
https://play.google.com/store/apps/details?id=de.andreas1724.bigdigitalclock&amp;hl=en

Just search the term 'clock' in google play and you will find many.