  We present a precise ground-based optical transmission spectrum of the
hot-Saturn HATS-5b ($T_{eq} =1025$ K), obtained as part of the ACCESS survey
with the IMACS multi-object spectrograph mounted on the Magellan/Baade
Telescope. Our spectra cover the 0.5 to 0.9 micron region, and are the product
of 5 individual transits observed between 2014 and 2018. We introduce the usage
of additional second-order light in our analyses which allows us to extract an
"extra" transit light curve, improving the overall precision of our combined
transit spectrum. We find that the favored atmospheric model for this
transmission spectrum is a solar-metallicity atmosphere with sub-solar C/O,
whose features are dominated by H$_2$O and with a depleted abundance of Na and
K. If confirmed, this would point to a "clear" atmosphere at the pressure
levels probed by transmission spectroscopy for HATS-5b. Our best-fit
atmospheric model predicts a rich near-IR spectrum, which makes this exoplanet
an excellent target for future follow-up observations with the James Webb Space
Telescope, both to confirm this H$_2$O detection and to superbly constrain the
atmosphere's parameters.
