RC circuit as clock source
You have basically already answered your own question. A simple R &amp; C circuit configured as a low pass or a high pass filter circuit is not capable of sustaining oscillation. For oscillation you need something that provides feedback from output to input of the circuit with 180 degree phase shift. Almost always that requires the circuit to contain some type of active component that adds gain so that the oscillation can be sustained. Without it any oscillation from a Time 0 application of the feedback will die out in a few cycles due to the lossy nature of the circuit.

If you do get an R &amp; C circuit to oscillate with some active components (transistor, MSFET, opamp, comparator, tube, valve) you can then use the resulting AC waveform as a clock signal should its amplitude, frequency and average voltage level meet the needs of the clock consumer. Voltage and amplitude characteristics are easily adjusted with some additional circuitry. Frequency can be changed by either changing R &amp; C values in the oscillator circuit or using counters and flip flops to divide the frequency to a lower value.