May I ask why? The time shows in the upper status bar, and most apps, (except games and videos) will show the upper status bar when open. If I saw a clock on an open app *and* the status bar clock, that would drive me nuts. Is there some reason you cannot see the status bar clock? Do you need to keep track of your screen time? Use digital wellbeing.
So, the short answer is, yes, it can be done, but only by keeping the clock on the upper status bar; yet also no, because unless an app includes its own clock there's no way to add it.
