From Longman Dictionary of Contemporary Englishclam up phrasal verbinformalTALK TO somebody to suddenly stop talking, especially when you are nervous or shy A sensitive child is likely just to clam up. → clam→ See Verb table
Examples from the Corpus
clam up• He always clams up when I ask him about his job -- do you think he's doing something illegal?• The police took her in for questioning, but she clammed up when they asked about her boyfriend. 