My prefix, proudest hunter, still stands guard o'er home and grain;
My suffix, firm foundation, from which space is realised;
My infix, part outstanding, leaves a gap for us to work.
But ye who seek me out, beware: for heroes fall, but not all rise.
EDIT: Let's add some hints. I've tried to combine clear factual hinting with additional clever wordplay, because that's the mindset I'm in here!
My foundation is in mathematics, which has definitely had some impact on where this riddle ended up.
I came up with the infix clue last; it's a little weak, and there's an argument for replacing it with whitespace.
The word itself is more obscure than I realised - I thought it was used in a fairly well-known bit of narrative theory, but it looks like while the concept is, the word itself might not be. That in mind, I'm not surprised it's all Greek to you.