  We present a measurement of the direct $CP$-violating charge asymmetry in
$D_s^\pm \to \phi \pi^{\pm}$ decays where the $\phi$ meson decays to $K^+K^-$,
using the full Run II data set with an integrated luminosity of 10.4 fb$^{-1}$
of proton-antiproton collisions collected using the D0 detector at the Fermilab
Tevatron Collider. The normalised difference $A_{CP}$ in the yield of $D_s^+$
and $D_s^-$ mesons in these decays is measured by fitting the difference
between their reconstructed invariant mass distributions. This results in an
asymmetry of $A_{CP} =\left[ -0.38 \pm 0.27 \right]%$, which is the most
precise measurement of this quantity to date. The result is consistent with the
standard model prediction of zero $CP$ asymmetry in this decay.
  Updated to include changes made after referees comments. Corrected an
affiliation in the author list.
