Frodo's Burden is a reference to the character Frodo Baggins from J.R.R. Tolkien's novel The Lord of the Rings. In the novel, Frodo is given the task of destroying the One RING, a powerful magical artifact that was created by the Dark Lord Sauron. The RING has the power to enslave all of Middle-earth, and so Frodo must carry it to the fires of Mount Doom in order to destroy it. This task is referred to as Frodo's Burden, and the answer to the crossword clue is 'RING'.