Magnetic reconnection is thought to lie at the heart of energy release in solar flares, but the process is not yet fully understood. We examine spectropolarimetric data from the CoMP coronagraph, acquired a few hours into the evolution of the September 10th 2017 X8.2-class flare. We find a striking and spatially coherent low polarisation structure, aligned with the hot plasma sheet observed in EUV. By elimination, we find the significant depolarisation to be a result of small-scale sub-pixel magnetic structure along the plasma sheet, consistent with theory of reconnection instabilities. This interpretation of ongoing reconnection is supported by further Hinode/EIS observations and AIA DEMs, from well beyond the impulsive phase of the flare. The plasma sheet remains visible in CoMP linear polarisation over a day into the flare’s evolution, several hours after its last appearance in EUV. We conclude that polarisation measurements with new coronagraphs, such the DKIST CRYO-NIRSP instrument, will further enhance our understanding of magnetic reconnection during eruptive flares.