Abstract The Transiting Exoplanet Survey Satellite (TESS) produces a large number of single-transit event candidates, since the mission monitors most stars for only ∼27 days. Such candidates correspond to long-period planets or eclipsing binaries. Using the TESS Sector 1 full-frame images, we identified a 7750 ppm single-transit event with a duration of 7 hours around the moderately evolved F-dwarf star TIC-238855958 (Tmag=10.23, Teff=6280±85 K). Using archival WASP photometry we constrained the true orbital period to one of three possible values. We detected a subsequent transit-event with NGTS, which revealed the orbital period to be 38.20 d. Radial velocity measurements from the CORALIE Spectrograph show the secondary object has a mass of M2= 0.148 ± 0.003 M⊙, indicating this system is an F-M eclipsing binary. The radius of the M-dwarf companion is R2 = 0.171 ± 0.003 R⊙, making this one of the most well characterised stars in this mass regime. We find that its radius is 2.3-σ lower than expected from stellar evolution models.