I think it is perfectly safe to drive. It is an anti rattle clip. We have had people on here who did not even know that their car was supposed to have these clips.
Unless you are having some sort of brake problem, it should be fine to drive until you can order one. If you are really concerned, you can check the caliper after a drive, and see if it is hotter than the other side (I am assuming you have the clip on the other side). If it is a lot hotter, then the pad may be dragging on the side with the missing clip, which is not a great situation.
But, for the most part, it should not cause anything more than some excess caliper noise.