Answer: clamsup

AnswerCrossword Clue
CLAMSUPForgoes conversation
CLAMSUPDecides to stop talking
CLAMSUPRefuses to talk
CLAMSUPRefuses to talk: Slang.
CLAMSUPStops talking: Slang.
CLAMSUPRefuses to spill the beans
CLAMSUPButtons one's lip
CLAMSUPIs tight-lipped
CLAMSUPStops talking suddenly
CLAMSUPStops yapping
CLAMSUPZips one's lip