We study the "prepeak" appearing in the static structure factor of the molecular glass-former m-toluidine by means of neutron scattering experiments and Monte Carlo simulations. The occurrence of this prepeak is interpreted as resulting from spatial organization of the molecules that goes beyond the usual short range liquid order and has a typical length scale of several molecular diameters. The origin of this phenomenon, as well as its specific temperature and density dependence, is explained by the competition between hydrogen-bonding interactions that tend to favor clustering and steric hindrance between aromatic rings that limits the extension of the H-bond network. Finally, effects of such clustering on the relaxational properties of the liquid and on the glass transition are discussed.