  The azimuthal correlation between the planes formed by the vectors
  $(\vec{p}_\ell,\vec{p}_{X_b})$ and $(\vec{p}_\ell,\vec{P}_t)$ in the
semileptonic rest frame decay of a polarized top quark $t(\uparrow) \to X_b +
l^+ + \nu_\ell$ belongs to a class of polarization observables involving the
top quark which vanish at the Born term level in the standard model. We
determine the next--to--leading order QCD corrections to the afore-mentioned
azimuthal correlation and compare the result to the corresponding contribution
of a non--standard--model right--chiral quark current.
