Q. How do you form a possessive of a “one of the” phrase? For example, a shout belonging to “one of the guards.” Placing the apostrophe at the end of “guards” seems to make multiple guards possess the shout. “Guard’s” seems to make it one of the shouts of a single guard. But if there are multiple guards, and one is shouting . . . where does the apostrophe go?
A. Our recommendation would be to rephrase to avoid the possessive—for example, by referring to “a shout from one of the guards.” Because you’re right, an apostrophe by itself won’t convey your intended meaning.
For example, you could write this: “One of the guards’ shouts could be heard above the din.” Restating that sentence reveals its meaning: “Of the guards’ shouts, one could be heard above the din.” There are multiple shouts from multiple guards (plural possessive), and one of these shouts in particular could be heard above the din—which is not, judging from your question, what you intend to say.
And you’re right about the version with apostrophe s, as in “one of the guard’s shouts.” As you say, that would suggest one shout from a single guard who is shouting—or maybe, depending on context, the shouts belonging to one of the guards. These are also not what you mean.
Again, rephrasing to avoid the possessive is your best bet. For a related scenario involving the phrase one of and verb agreement, see CMOS 5.62.