  We investigate the low temperature magnetic properties of a $S=\frac{5}{2}$
Heisenberg kagom\'e antiferromagnet, the layered monodiphosphate
Li$_9$Fe$_3$(P$_2$O$_7$)$_3$(PO$_4$)$_2$, using magnetization measurements and
$^{31}$P nuclear magnetic resonance. An antiferromagnetic-type order sets in at
$T_{\rm N}=1.3$ K and a characteristic magnetization plateau is observed at 1/3
of the saturation magnetization below $T^* \sim 5$ K. A moderate $^{31}$P NMR
line broadening reveals the development of anisotropic short-range correlations
within the plateau phase concomitantly with a gapless spin-lattice relaxation
time $T_1 \sim k_B T / \hbar S$, which both point to the presence of a
semiclassical nematic spin liquid state predicted for the Heisenberg kagom\'e
antiferromagnetic model.
