Clue: Plan to surprise carol singer, after extra rest (3,2,4)
Answer: LIE IN WAIT
Explanation: Definition: Plan to surprise
LIE IN (an extra rest) + WAIT (carol-singer). Apparently carol-singers were called 'Waits' because they only sang on Christmas Eve, which was known as Waitnight or Watchnight, perhaps in reference to the shepherds who watched their flock by night