I guess the "off the clock" means either of two things:
1) Fired for something that occurred during off-hours, not on the job.
2) Being terminated while you aren't technically punched in.
In both instances your employment can be terminated unless you have a contract of employment or other remedy at all. As Betty3 pointed out, your relationship is "at will" which means it's the employer's will that matters, whether he wants to keep you employed or not. He or she can terminate your working relationship for no reason at all, e.g. he or she just wants to find someone else.