What's it called?

[Allan]

The piece of plastic at the top of the playfield glass.
Part of it goes over the glass and the bottom bit goes under the glass to form a seal, of sorts.
Once I know what it is I'll need to get one as mine is broken.

 

[Spandangler]

Glass channel. They're about £7 from all good parts suppliers ;-)

 

[mark9]

Or try "Rear moulding", usually comes in two sizes, widebody or standard.