  X-ray eclipse mapping is a promising modelling technique, capable of
constraining the mass and/or radius of neutron stars (NSs) or black holes (BHs)
in eclipsing binaries and probing any structure surrounding the companion star.
In eclipsing systems, the binary inclination, $i$, and mass ratio, $q$ relate
via the duration of totality, $t_{e}$. The degeneracy between $i$ and $q$ can
then be broken through detailed modelling of the eclipse profile. Here we model
the eclipses of the NS low-mass X-ray binary Swift J1858.6$-$0814 utilising
archival NICER observations taken while the source was in outburst. Analogous
to EXO 0748$-$676, we find evidence for irradiation driven ablation of the
companion's surface by requiring a layer of stellar material to surround the
companion star in our modelling. This material layer extends $\sim 7000 -
14000$ km from the companion's surface and is likely the cause of the extended,
energy-dependent and asymmetric ingress and egress that we observe. Our fits
return an inclination of $i \sim 81^{\circ}$ and a mass ratio $q \sim 0.14$.
Using Kepler's law to relate the mass and radius of the companion star via the
orbital period ($\sim$ 21.3 hrs), we subsequently determine the companion to
have a low mass in the range $0.183 M_{\odot} \leq M_{cs} \leq 0.372 M_{\odot}$
and a large radius in the range $1.02 R_{\odot} \leq R_{cs} \leq 1.29
R_{\odot}$. Our results, combined with future radial velocity amplitudes
measured from stellar absorption/emission lines, can place precise constraints
on the component masses in this system.
