  The stability of the magnetization $m=1/3$ plateau phase of the XXZ spin-1/2
Heisenberg chain with competing interactions is investigated upon switching on
a staggered transverse magnetic field. Within a bosonization approach, it is
shown that the low-energy properties of the model are described by an effective
two-dimensional XY model in a three-fold symmetry-breaking field. A phase
transition in the three-state Potts universality class is expected separating
the $m=1/3$ plateau phase to a phase where the spins are polarized along the
staggered magnetic field. The Z$_3$ critical properties of the transition are
determined within the bosonization approach.
