How would I go about building the clock applet? Source, apps needed?
Which clock? The one in the panel - or the time clock on the desktop?
If the panel - is it the default clock - or the clock from the calendar applet?
Panel, default clock.
So that applet is one of the default applets from upstream budgie-desktop.
You’ll need to “hack” the code - How to hack the budgie desktop on ubuntu
The clock applet code is here - https://github.com/solus-project/budgie-desktop/tree/master/src/applets/clock