Is there a screen clock for the IPad?

Try searching for "clock" in the Apple App Store. Here you will find many Apps from which to choose - some of which will be free, some requiring a payment - and others that may have an in-App purchase to enable specific features.

You are free to choose an App that meets your needs - and/or your pocket.

The built in Clock app can be used to add a clock widget which displays the time. You could also setup a widget that displays the time indifferent time zones around the world. By default the time is shown on the lock screen.

Press and hold on an empty part of the iPad display to activate jiggle mode, select Edit, select Add Widget, and select the Clock widget.