According to the official website, CLAMP
made a movie of VOCALOID Hatsune Miku
song and uploaded it on NicoNico Douga.
Mugen no Yami -echo of the past (Niconico)
The song was originally composed by a game music artist Hosoe Shinji ("Pop'n Music", "beatmania IIDX"). CLAMP found the song when they were browsing Niconico douga and collaborated with him through the meditation of Mihara Ichiro, the vice president of a game company ARIKA (and the model of CLAMP's character Mihara Ichiro
CLAMP worked on the movie in their spare time and it took a month to complete it. They said "We work sincerely, and play seriously."
Source: CLAMP official website