  We present the results of ALMA spectroscopic follow-up of a $z=6.765$
Lyman-$\alpha$ emitting galaxy behind the cluster RXJ1347-1145. We report the
detection of [CII]158$\mu$m line fully consistent with the Lyman-$\alpha$
redshift and with the peak of the optical emission. Given the magnification of
$\mu=5.0 \pm 0.3$ the intrinsic (corrected for lensing) luminosity of the [CII]
line is $L_{[CII]} =1.4^{+0.2}_{-0.3} \times 10^7L_{\odot}$, which is ${\sim}5$
times fainter than other detections of $z\sim 7$ galaxies. The result indicates
that low $L_{[CII]}$ in $z\sim 7$ galaxies compared to the local counterparts
might be caused by their low metallicities and/or feedback. The small velocity
off-set ($\Delta v = 20_{-40}^{+140} \rm km/s$) between the Lyman-$\alpha$ and
[CII] line is unusual, and may be indicative of ionizing photons escaping.
