The 21,728th page of 1989's 20-volume Oxford English Dictionary

END

Think of where you’d find the last word in a massive book—or the final entry in any complete list.

This clue plays on the idea of the Oxford English Dictionary being a massive reference work with a clear beginning and end. The 20-volume 1989 edition had 21,728 pages, so the last page would logically be the "end"—both literally as the final page and as the word "end" itself. It’s a clever way to turn a factual detail into a pun, since "end" fits as the answer in two different senses.