Answer:
The edge of earth and bread.
Long and thin, red within, with a nail at the end.
Take one royal word in the plural and make it singular by adding one letter
The more you look at it, The less you see.
Built of metal or wood to divide. It will make us good neighbors, if you stay on your side. What is it?
Though not a plant, has leaves. Though not a beast, has spine. Though many wouldn't need this thing, It's more valuable than wine.