If you want a project it could be fixable for a while but if you just want a belt it's way more worth it to just buy a better quality one if possible.
If you happen to have materials at hand and want to try and fix it I'd start by punching 2-3 very small holes on either side of the rip to sew through. Try and avoid having them so big or so close to the holes you're going to use as a belt because this might start another tear.
Wrap the rip with a sturdy fabric possibly even a very dense batting as that will hold together well. If you use fabric, tuck the ends under. If you use batting, you don't have to worry about that. Wrap it on both sides and sew it through the holes.
Turn it over and sew the gap shut either using invisible stitches or not! It's on the back so as long as it holds together it'll be fine. Keep in mind this will still eventually come apart but it might hold you over until you can get a new or better one.
You don't have to but you could also use fabric paint on the entire patch to help it match the leather color and this would also seal the stitches in so it would last a bit longer. Good luck!