  Large widefield surveys make possible the serendipitous discovery of rare
sub-classes of pulsars. One such class are "spider"-type pulsar binaries,
comprised of a pulsar in a compact orbit with a low-mass (sub)stellar
companion. In a search for circularly-polarized radio sources in ASKAP Pilot
Survey observations, we discovered highly variable and circularly polarized
emission from a radio source within the error region of the $\gamma$-ray source
{4FGL}~J1646.5$-$4406. The variability is consistent with the eclipse of a
compact, steep-spectrum source behind ablated material from a companion in a
$\sim 5.3\,$h binary orbit. Based on the eclipse properties and spatial
coincidence with {4FGL} J1646.5$-$4406, we argue that the source is likely a
recycled pulsar in a "redback" binary system. Using properties of the eclipses
from ASKAP and Murchison Widefield Array observations, we provide broad
constraints on the properties of the eclipse medium. We identified a potential
optical/infra-red counterpart in archival data consistent with a variable
low-mass star. Using the Parkes Radio Telescope "Murriyang" and MeerKAT, we
searched extensively for radio pulsations but yielded no viable detections of
pulsed emission. We suggest that the non-detection of pulses is due to
scattering in the intra-binary material, but scattering from the ISM can also
plausibly explain the pulse non-detections if the interstellar dispersion
measure exceeds $\sim$600$\,$pc$\,$cm$^{-3}$. Orbital constraints derived from
optical observations of the counterpart would be highly valuable for future
$\gamma$-ray pulsation searches, which may confirm the source nature as a
pulsar.
