  We measure the harmonic-space auto-power spectrum of the galaxy overdensity
in the LOFAR Two-metre Sky Survey (LoTSS) First Data Release and its cross
correlation with the map of the lensing convergence of the cosmic microwave
background (CMB) from the Planck collaboration. We report a $\sim5\sigma$
detection of the cross-correlation. We show that the combination of the
clustering power spectrum and CMB lensing cross-correlation allows us to place
constraints on the high-redshift tail of the redshift distribution, one of the
largest sources of uncertainty in the use of continuum surveys for cosmology.
Our analysis shows a preference for a broader redshift tail than that predicted
by the photometric redshifts contained in the LoTSS value added catalog, as
expected, and more compatible with predictions from simulations and
spectroscopic data. Although the ability of CMB lensing to constrain the width
and tail of the redshift distribution could also be valuable for the analysis
of current and future photometric weak lensing surveys, we show that its
performance relies strongly on the redshift evolution of the galaxy bias.
Assuming the redshift distribution predicted by the Square Kilometre Array
Design simulations, we use our measurements to place constraints on the linear
bias of radio galaxies and the amplitude of matter inhomogeneities $\sigma_8$,
finding $\sigma_8=0.69^{+0.14}_{-0.21}$ assuming the galaxy bias scales with
the inverse of the linear growth factor, and $\sigma_8=0.79^{+0.17}_{-0.32}$
assuming a constant bias.
