I would say: The clock was always one hour behind. The clock was always one hour behind the actual time. The clock was always one hour late. The clock was always one hour slow. You can also use "always ran" for any of the above examples. The clock always ran one hour slow I wouldn't say "original" because what you mean is that the clock is one hour behind the correct time.

Thank you!

Yes, it's correct.

It’s not clear what you mean by ‘original time’.