What is Clock ?
Clock is (noun) an instrument which shows the time The station clock is always right. Your clock is 5 minutes slow. The office clock is fast. The clock has stopped – it needs winding up.(verb) to clock in or on to arrive for work and register by putting a card into a timing machine to clock out or off to leave work and register by putting a card into a timing machine
source: Easier English, Student Dictionary Upper Intermediate Level