Possible Clues:

• Refuse to talk- like shellfish?
• Become closemouthed
• Become silent
• Not say a word
• Evidence of traffic wardens outside university makes one stop talking
• Abruptly stop talking (colloq.)
• Little Malcolm might be made to stop talking
• Refuse to talk, like a shellfish
• By ine-fifty in the morning I may be out of bed, but I won't give you any information
• Don't talk of university when restricted by immobiliser