This is a clip that can hold a fold in the waistband of a clothing item that is a bit too big. This is a clip that can hold a fold in the waistband of a clothing item that is a bit too big. 
This only works for relatively thin clothes, like skirts, though. For anything thicker like jeans you'd need a belt anyway. 

Note, that the side where I have my thumb in the picture goes on the inside. Otherwise it would just turn outside. The side is rounded out, so it should not be too uncomfortable. You maybe need to go over with a blow torch to get rid of any scratchy bits, but other than that, it's fine. 

I added a sliced file. It is on the core one, but if you have an other printer, you should still look at it, as the orientation (and supports) and perimeters are quite important for strength. If you'd print it standing up, it will probably brake. 
I used PLA and it seems to hold up good enough. But i'd still recommend using PETG or even ABS/ASA, if you have any. Just don't use Silk PLA…