clam_up
1. [ verb ] refuse to talk or stop talking; fall silent
Synonyms: shut_up be_quiet belt_up button_up dummy_up keep_mum close_up
Examples:

"The children shut up when their father approached"

Related terms: open_up
Similar spelling:   climb_up
  clean_up
  cleanup
  clump
  clomp
  clamp
  climb_on
  Clampitt
  climb
  clampdown