I've had a couple of this style of plastic clip crack. Wrapping it in thread and then applying super glue seems to work pretty well. I also tried wrapping with fishing line, but it came undone pretty quickly. Just wanted to share the tip! Now if only I had a fix for when the prongs on the other half break off.
If you have an Osprey bag they'll send you replacement clips for free
Timbuk2 as well, lots of gear stores will have them too.