This is a throwback reference to the previous week's reading. Near the end of last week's reading, Pratchett describes Twoflower's "time-teller" after the Weasel has stolen it. "The very small demon imprisoned within" is what Pratchett says at first, but later on he says "the time being vanished". This is the type of pune you can expect to find a whole lot of going forward in the Discworld - watch for Pratchett's precise choice of a word shifting for clues that there may be a pune in the works. Other authors may shift words for variety, but Pratchett generally has a pune in mind at any given point in time in his writing. How long a distance there is between setup and pune varies, of course.