  By applying measurements of the dielectric constants and relative length
changes to the dimerized molecular conductor
$\kappa$-(BEDT-TTF)$_2$Hg(SCN)$_2$Cl, we provide evidence for order-disorder
type electronic ferroelectricity which is driven by charge order within the
(BEDT-TTF)$_2$ dimers and stabilized by a coupling to the anions. According to
our density functional theory calculations, this material is characterized by a
moderate strength of dimerization. This system thus bridges the gap between
strongly dimerized materials, often approximated as dimer-Mott systems at 1/2
filling, and non- or weakly dimerized systems at 1/4 filling exhibiting charge
order. Our results indicate that intra-dimer charge degrees of freedom are of
particular importance in correlated $\kappa$-(BEDT-TTF)$_2$X salts and can
create novel states, such as electronically-driven multiferroicity or
charge-order-induced quasi-1D spin liquids.
