Yes, ‘off the clock’ can mean I’m still at work, but on an unpaid break. Normally Taxi drivers would say something like this. I don’t think an office worker would.

You can, but I would say that the meaning is a little more specific than just "at work" or "not at work." If you're "on the clock," then you're currently being paid for your time. You shouldn't do things that aren't work-related, like watch funny videos on YouTube, during the time that your company is paying you to work. "Off the clock" refers to time that you are NOT being paid for (especially if you're still at your work location during this time). For example, if your boss caught you relaxing in the break room, you could explain, "It's okay, I'm off the clock. I'm just waiting for my friend to give me a ride home." In this situation, it's fine for you to relax and not work, since you're not being paid for your time right now. At some jobs, you're not allowed to do work while you're "off the clock," because the company could get in trouble if someone claimed that the company was not paying people for all their work, or if an employee got injured while working when they weren't supposed to be working. So your boss might tell you, "Once you're off the clock, you can't hang around in the work area. You have to go home." In short, "off the clock" doesn't always mean that you're "finished with work," although it can mean that in certain contexts. More generally, it just means that your time/work is not currently being paid for by the company.

Yes, it could mean that one is off duty, relaxing or not paid for a work being currently done.

Absolutely!