  The magnetic subsystem of nabokoite, KCu$_7$(TeO$_4$)(SO$_4$)$_5$Cl, is
constituted by copper ions forming a buckled square kagom\'e lattice decorated
by quasi-isolated ions. This combination determines peculiar physical
properties of this compound evidenced in electron spin resonance (ESR)
spectroscopy, dielectric permittivity $\varepsilon$, magnetization $M$ and
specific heat $C_p$ measurements. At lowering temperature, the magnetic
susceptibility $\chi = M/H$ passes through a broad hump inherent for
low-dimensional magnetic systems at about 150 K and a sharp peak at
antiferromagnetic phase transition at $T_N = 3.2 $K. The $C_p(T,H)$ curves
demonstrate additional peak-like anomaly at $T_{peak}= 5.7$K robust to magnetic
field. The latter can be ascribed to low-lying singlet excitations filling the
singlet-triplet gap in magnetic excitation spectrum of the square kagom\'e
lattice [J.Richter, O.Derzhko and J.Schnack, Phys. Rev. B \textbf{105} (2022)
144427]. ESR spectroscopy provides indications that antiferromagnetic structure
below $T_N$ is non-collinear. Separate issue is the observation of
antiferroelectric-type behavior in $\varepsilon$ at low temperatures, which
tentatively reduces the symmetry and partially lifts frustration of magnetic
interactions of decorating copper ions with buckled square kagom\'e lattice.
These complex thermodynamic and resonant properties signal the presence of two
weakly coupled magnetic subsystems in nabokoite, namely a spin-liquid in square
kagom\'e lattice layers and an antiferromagnet represented by decorating ions.
