(literally) Displayed on the dial of a clock (timepiece).
(idiomatic) Working at one's job; occupied in some manner during one's hours of remunerated employment.
(idiomatic) During one's official working hours; in or into a position of remunerated employment.
(idiomatic) Of a taxicab, engaged for hire; displayed numerically as time or fare on the meter of a taxicab.
(sports) In the official time remaining in a game or other sporting event.
(sports) In the official time expired in a game or other sporting event.
(chiefly sports drafts) Under scrutiny due to having to make a decision or produce results within a set period of time.
(of a motor vehicle) Displayed numerically on the mileage or kilometric gauge.
Remunerated per unit of time.
Alas, the delivery boy is on the clock; he could and would work much faster on commission!