  Context: Transmission spectroscopy has become a prominent tool for
characterizing the atmospheric properties on close-in transiting planets.
Recent observations have revealed a remarkable diversity in exoplanet spectra,
which show absorption signatures of Na, K and $\mathrm{H_2O}$, in some cases
partially or fully attenuated by atmospheric aerosols. Aerosols (clouds and
hazes) themselves have been detected in the transmission spectra of several
planets thanks to wavelength-dependent slopes caused by the particles'
scattering properties. Aims: We present an optical 550 - 960 nm transmission
spectrum of the extremely irradiated hot Jupiter WASP-103b, one of the hottest
(2500 K) and most massive (1.5 $M_J$) planets yet to be studied with this
technique. WASP-103b orbits its star at a separation of less than 1.2 times the
Roche limit and is predicted to be strongly tidally distorted. Methods: We have
used Gemini/GMOS to obtain multi-object spectroscopy hroughout three transits
of WASP-103b. We used relative spectrophotometry and bin sizes between 20 and 2
nm to infer the planet's transmission spectrum. Results: We find that WASP-103b
shows increased absorption in the cores of the alkali (Na, K) line features. We
do not confirm the presence of any strong scattering slope as previously
suggested, pointing towards a clear atmosphere for the highly irradiated,
massive exoplanet WASP-103b. We constrain the upper boundary of any potential
cloud deck to reside at pressure levels above 0.01 bar. This finding is in line
with previous studies on cloud occurrence on exoplanets which find that clouds
dominate the transmission spectra of cool, low surface gravity planets while
hot, high surface gravity planets are either cloud-free, or possess clouds
located below the altitudes probed by transmission spectra.
