1. The clip is an Anti Rattle clip.
2. It is "safe to drive" without it,but the brake pad will rattle, and get worse over time.
3. If the other side is O.K., use that to determine how to put it back.
4. Part #? Don't know.
If the clip was put on wrong, it may of kept the caliper from moving properly. When the clip fell off, it allowed the caliper to move and that may of restored the brake feel.
