Answer: clammed

AnswerCrossword Clue
CLAMMED... up (kept mum)
CLAMMEDDug for quahogs
CLAMMEDShut (up)
CLAMMEDWent silent (with "up")
clammedCLAM, to gather the shellfish, clams
clammedAbruptly stop talking, either for fear of revealing a secret or from shyness
clammedDig for or collect clams