1. Currently engaged in work during one's normal working day or assigned hours. I'd love to go have a beer with you, but as you can see by the uniform I'm wearing, I'm actually on the clock right now.

2. Of a taxi (and by extension, its driver), currently engaged to carry a passenger and having an active meter. Sorry, but I can't stay to chat. I've got a taxi waiting outside that's still on the clock!

3. Currently displayed on a motor vehicle's speedometer or odometer (also called a mileometer). I can't believe I got a ticket for speeding—I was only going 50 on the clock! This car may have 200,000 kilometers on the clock, but it still runs as smooth as ever.