  The Transiting Exoplanet Survey Satellite (\tess) produces a large number of
single-transit event candidates, since the mission monitors most stars for only
$\sim$27\,days. Such candidates correspond to long-period planets or eclipsing
binaries. Using the \tess\ Sector 1 full-frame images, we identified a
7750\,ppm single-transit event with a duration of 7\,hours around the
moderately evolved F-dwarf star \tic\ (Tmag=10.23, \teff=6280$\pm{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 $M_2$=
$0.148\pm{0.003}$\,M$_{\odot}$, indicating this system is an F-M eclipsing
binary. The radius of the M-dwarf companion is $R_2$ =
$0.171\pm{0.003}$\,R$_{\odot}$, making this one of the most well characterised
stars in this mass regime. We find that its radius is 2.3-$\sigma$ lower than
expected from stellar evolution models.
