[TheGreatUnknown]

I removed the default clock widget, the one that's on the lock screen. I deleted it off my home screen by accident, how on earth do I get it back and how do I customize it?!

[Kelly Burby]

Well just go to the menu and look for the clock in the widgets, once you find it you just need to drag and drop it to your home screen !!

[Kibbster]

You can't delete the widget on the lock screen (you can remove the notifications, but not the clock itself.)
Do you mean the homescreen widget?
Are you using the stock blinkfeed launcher?

If so like Kelly said above, long press on the screen select widgets and then drag the widget you want onto the screen.
I can't repeat the steps for you because I'm not running the Blinkfeed launcher so can't use the clock widget.