This is a polished remake of a cryptic by Emily Cox and Henry Rathvon, which appeared in the Wall Street Journal last year. The instructions are as follows:
When all clue answers are entered in the grid, the inner box will be surrounded by 16 different letters of the alphabet. Use the remaining 10 letters of the alphabet to construct a three-by-three
crossword for that inner box—with a pair of letters occupying the central square.
(Alternatively, you may fill the box with X's if you wish to skip the final step.)