“On the clock”: to be working/getting paid
The police officer stopped to have dinner on his break. The waitress offered him a beer. He said, “No thanks, I’m on the clock.”
“Off the clock”: not at work/off duty/not being paid to work
Rebecca works at a grocery store. She had just finished her shift and was leaving for the day.
On her way out, a coworker asked for her help. Rebecca said, “Sorry I can’t help, I’m off the clock.”