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

Discovery of radio eclipses from 4FGL J1646.5$-$4406: a new candidate redback pulsar binary