time - What is a clock?

Relationalists love to define time operationally as what clocks measure, but this begs the question of what counts as a clock. Clearly, it's a measuring instrument and what it measures is supposedly time. We need to avoid circular definitions of time. There are many possible measuring devices, and most don't measure time. Which of them counts as a clock? To which one has to answer it has to give a linear real number as a pointer reading, it orders events correlationally, etc etc has all the properties we demand of time. This is circular.