Elevenses is real. It's a late morning snack, served around (surprise) eleven in the morning. You usually have coffee or tea, and cakes or pastries. The term is now kind of antiquated, and I don't think it sees much currency, but Tolkien definitely did not invent it. It dates back to at least the late nineteenth century.
The term sounds quaint, but it's equivalent to a mid-morning coffee break, which isn't uncommon in the US.
In the film version The Fellowship of the Ring, Pippin refers to both elevenses and second breakfast, as well as luncheon, tea, dinner, and supper. That's six, plus the breakfast they already had makes seven. Even Tolkien thought that hobbits ate only six meals a day, and having both second breakfast and elevenses is probably redundant (especially since second breakfast traditionally includes sausages as well, and is served a mere half-hour before elevenses).