In Act One, just as John Proctor enters the room, Miller provides some background information. He says, in part, "In Proctor's presence a fool felt his foolishness instantly -- and a Proctor is always marked for calumny therefore." What this line means is that Proctor was the kind of man who inspired the resentment of those who were less morally upright and/or less intelligent than he. And because of this resentment, he is often subject to untrue statements that others make to damage his otherwise sterling reputation. However, because Miller says "a Proctor" instead of just "Proctor," the implication is that this description doesn't only apply to John Proctor alone, perhaps that it can apply to Elizabeth as well.
Elizabeth's spotless reputation has surely inspired Abigail Williams's resentment of her. Likewise, Abigail has made many disparaging and untrue statements about Elizabeth Proctor, just as some others do about John. For example, she tells her uncle that she was dismissed from the Proctors' service because Elizabeth wanted a "slave," and Abigail refused to be one. Just as John's manner and honesty provoke others who are less honest and forthright to slander him, so do Elizabeth's. Miller's words could be a bit of foreshadowing, then.