Hi,
A rivet or Chicago screw will do it. If there's a way to fit the clip so that the structure of the holster takes the weight and the brunt of the work, and the rivet or screw simply keeps the clip in place, it'll be more secure.
For example, if the clip is sewn into a slot in the holster body and then the rivet set through the leather on one side, through the clip in the middle and out through the leather on the other side.
Cheers,
Badger