If you use a Clock to represent a complex series of operations to achieve a goal, you must also establish when that goal will be irredeemably lost: does it take a single failed Check, or is there a parallel "failure" Clock that fills whenever a character rolls poorly? The Game Master determines this on a case-by-case basis, but must make sure to inform the Players as they approach the challenge.
Characters can also take action to slow a Clock's advance and erase some of the filled sections, eventually bringing it back to 0 — the method is identical to that of filling a Clock. When a Clock reaches 0, the Game Master may have the threat removed or simply keep it lurking in the shadows.
Example: As the heroes fight the powerful sorceress Chrona, her ritual to open a rift to the Demon Realm draws to its conclusion. The Player Characters must balance their actions between defeating the sorceress and slowing down the progress of the ritual, a ten-sections Clock that would end the scene in an automatic defeat.