  We study magnetic behaviour of the Yb$^{3+}$ ions on a frustrated pyrochlore
lattice in the spinel {\CYS}. The crystal-electric field parameters deduced
from high-energy inelastic neutron scattering reveal well-isolated ytterbium
ground state doublet with a weakly Ising character. Magnetic order studied by
powder neutron diffraction evolves from the $XY$-type antiferromagnetic
$\Gamma_5$ state to a splayed ice-like ferromagnet (both with k=0) in applied
magnetic field with $B_c$=3 T. Low-energy inelastic neutron scattering
identifies weakly dispersive magnetic bands around 0.72 meV starting at
$\mid\bf{Q}\mid$ = 1.1 \AA$^{-1}$~ at zero field, which diminish with field and
vanish above 3 T. We explain the observed magnetic behaviour in framework of
the nearest-neighbour anisotropic exchange model for effective $S=1/2$ Kramers
doublets on the pyrochlore lattice. The estimated exchanges position the {\CYS}
spinel close to the phase boundary between the $\Gamma_5$ and splayed
ferromagnet states, similar to the Yb-pyrochlores suggesting an important role
of the competition between these phases.
