
We usually don’t tell people to clam up. We use it for situations where people have something to say, or can contribute to a discussion, but for some reason decide to remain silent. Or when someone who is speaking about something suddenly stops talking, as if they’ve already said too much: “He was telling us what happened at the nightclub the night before, but when his brother walked in the room, he clammed up and said he couldn’t remember anything.“ (=it’s obvious he knew more, but decided not continue talking)