Leaked by 2ch, CLAMP's new manga looks like it will appear in Shueisha's monthly magazine, Jump Square, in December's issue. The magazine will be released November 4th.
Jump Square is a shonen magazine that contains familiar titles such as Claymore, Shiki, D.Gray-man and Tegami Bachi. starlady38 lets me know that the image also says that there will be color pages. :)
Feel free to comment with more information.
ETA: "The manga is part of the "Supreme Yomikiri Series" (Supreme One-Shot Series)" --ANN
JUMP SQ's splash promo
Natalie.mu's news post