  We report high resolution neutron scattering measurements on the triangular
lattice Mott insulator Na$_x$CrO$_2$ ($x$=1) which has recently been shown to
exhibit an unusually broad fluctuating crossover regime extending far below the
onset of spin freezing ($T_c\sim$41K). Our results show that below some
crossover temperature ($T\sim0.75T_c$) a small incommensuration develops which
helps resolve the spin frustration and drives three-dimensional magnetic order
supporting coherent spin wave modes. This incommensuration assisted dimensional
crossover suggests that inter-layer frustration is responsible for stabilizing
the rare 2D correlated phase above 0.75$T_c$. In contrast to the host compound
of 2D cobaltate superconductor such as Na$_x$CoO$_2$ ($\xi_c>50$\AA), no
magnetic long-range order is observed down to 1.5K ($\xi_c<16$\AA).
