Can you apply as a flight attendant if you lose one of your teeth at the back?

Yes. You are able to work as a flight attendant even if you loose a tooth in the back of your mouth.

The tooth issue in regards to flying only applies to jet fighter pilots that endure more than 8 G (8 times their own weight) in certain exercises. If you have tooth fillings and experience this amount ogf gravity repeatedly the fillings may fall out.