I take it you're talking about the windshield? Can you just push it back in place?
If you have to glue it, I recommend using white glue, also known as PVA glue. Here in the States, Elmer's is one brand. I do not recommend using cyanoacrylate (CA) glue, also known as super glue. As CA glue cures, the fumes it gives off can damage the clear part, etching it and producing fogging. Granted, this piece looks like it's open enough to allow air to flow, but I wouldn't want to risk it.
But if the clear piece is flexible enough, I would just try to pop it back into place, especially if that's how it was assembled in the first place. That is, is that piece glued in already, or is it popped in, and held in place by friction?
Prost!
Brad