  Radio relics at the peripheries of galaxy clusters are tracers of the elusive
cluster merger shocks. We report the discovery of a single radio relic in the
galaxy cluster PLCK G200.9-28.2 ($z=0.22$, $M_{500} = 2.7\pm0.2 \times 10^{14}
M_{\odot}$) using the Giant Metrewave Radio Telescope at 235 and 610 MHz and
the Karl G. Jansky Very Large Array at 1500 MHz. The relic has a size of $\sim
1 \times 0.28$ Mpc, an arc-like morphology and is located at 0.9 Mpc from the
X-ray brightness peak in the cluster. The integrated spectral index of the
relic is $1.21\pm0.15$. The spectral index map between 235 and 610 MHz shows
steepening from the outer to the inner edge of the relic in line with the
expectation from a cluster merger shock. Under the assumption of diffusive
shock acceleration, the radio spectral index implies a Mach number of
$3.3\pm1.8$ for the shock. The analysis of archival XMM Newton data shows that
PLCK G200.9-28.2 consists of a northern brighter sub-cluster, and a southern
sub-cluster in a state of merger. This cluster has the lowest mass among the
clusters hosting single radio relics. The position of the Planck Sunyaev
Ze'ldovich effect in this cluster is offset by 700 kpc from the X-ray peak in
the direction of the radio relic, suggests a physical origin for the offset.
Such large offsets in low mass clusters can be a useful tool to select
disturbed clusters and to study the state of merger.
