  GRB 170817A, detected by Fermi-GBM 1.7\,s after the merger of a neutron star
(NS) binary, provides the first direct evidence for a link between such a
merger and a short-duration gamma-ray burst. The X-ray observations after GRB
170817A indicate a possible X-ray flare with a peak luminosity $L_{\rm peak}
\sim 2\times 10^{39}\,{\rm erg\,s}^{-1}$ near day 156. Here we show that this
X-ray flare may be understood based on a slim disc around a compact object. On
the one hand, there exists the maximal accretion rate $\dot M_{\rm max}$ for
the slim disc, above which an optically thick outflow is significant and
radiation from the disc is obscured. Based on the energy balance analysis, we
find that $\dot M_{\rm max}$ is in the range of $\sim 4\dot M_{\rm Edd}$ and
$\sim 21\dot M_{\rm Edd}$ when the angular velocity of the slim disc is between
$\rm (1/5)^{1/2}\Omega_K$ and $\rm \Omega_K$ (where $\dot M_{\rm Edd}$ is the
Eddington accretion rate and $\Omega_K$ is the Keplerian angular velocity).
With $\dot M_{\rm max}$, the slim disc can provide a luminosity $\sim L_{\rm
peak}$ for a compact object of $2.5 M_{\sun}$. On the other hand, if the merger
of two NSs forms a typical neutrino-dominated accretion disc whose accretion
rate $\dot M$ follows a power-law decline with an index $-1.8$ , then the
system must pass through the outflow regime and enter the slim disc in $\sim
11-355$ days. These results imply that a post-merger slim accretion disc could
account for the observed late-time $L_{\rm peak}$.
