It's a clip that holds the TM front sight on. The front sight is a post that goes through the slide.
Pin/clip the end of the post in place by wedging the clip on the top-inside of the slide.
Look close...you'll figure it out once you see it.
*btw....most disassembly guides skip over this part since you really don't want to take your front sight off much.
*PS. a very small drop of crazy glue once it's in place will help to ensure that it stays there (or else your front sight may go flying....not too common but that clip is all that's holding it on)