According to Anime News Network:
The April issue of Kodansha’s Nakayoshi magazine announced on Thursday that the members of the creative team CLAMP are publishing a new manga in their Cardcaptor Sakura franchise. The announcement does not list any details about the new work, and the magazine says it will announce which future issue will run the new manga when that is decided. The Japanese wording of the announcement indicates that the new manga is not a full-length series.
So it looks like it’s going to be a celebratory, temporary thing. Longer than an one-shot but shorter than a full series.
I already hear the OAD bells.
Update: according to Comic Natalie, it’s going to be a one-shot.