  The endohedral fullerene $^{15}\mathrm{N@C}_{60}$ has narrow electron
paramagnetic resonance lines which have been proposed as the basis for a
condensed-matter portable atomic clock. We measure the low-frequency spectrum
of this molecule, identifying and characterizing a clock transition at which
the frequency becomes insensitive to magnetic field. We infer a linewidth at
the clock field of 100 kHz. Using experimental data, we are able to place a
bound on the clock's projected frequency stability. We discuss ways to improve
the frequency stability to be competitive with existing miniature clocks.
