Wiktionary
clams up
vb. (en-third-person singular of: clam up)
Usage examples of "clams up".
When the admiral clams up, the cow chips are about to strike the windmill.
Makri pushes her head through the outside doorway and is about to say something when she notices the Pontifex and clams up.
The minute I walk in the door, she clams up and stays that way until I leave.