I think from looking it is the plastic retainer on the door panel side for that piece, where it attaches on the posterior portion to the door panel. I have that problem on both sides, you have to replace the panel. <sigh> There's not a replaceable clip there, it's a plastic piece that's molded in - unless you want to try to fab something, which would be difficult, as you can't permanently attach it. At least I don't think so.
<edit> I bought a pair of door panels on Egay, didn't realize until I got them that they had those plastic arms that hold them to the metal doorskin broken, too. What a waste of $200. Those door panels are a PITA.