  At cosmological distances, gravitational lensing can provide a direct
measurement of supermassive black hole (SMBH) masses irrespective of their
luminosities. Here, we directly estimate the mass of a SMBH in the brightest
cluster galaxy (BCG) of MACS J1149+2223.5 at $z=0.54$ through one of the
multiply-lensed images of a background spiral galaxy at $z=1.49$ projected
close to the BCG. In this particular image, an intrinsically compact region in
one of the spiral arms is lensed into an arc that curves towards the BCG
center. This arc has a radius of curvature of only $\sim$0."6, betraying the
presence of a local compact deflector. Its curvature is most simply reproduced
by a point-like object with a mass of $8.4^{+4.3}_{-1.8}\times10^{9}M_\odot$,
similar to SMBH masses in local elliptical galaxies having comparable
luminosities. The SMBH is noticeably offset by $4.4\pm0.3$ kpc from the BCG
light centre, plausibly the result of a kick imparted $\sim2.0\times10^7$ years
ago during the merger of two SMBHs, placing it just beyond the stellar core. A
similar curvature can be produced by replacing the offset SMBH with a compact
galaxy having a mass of $\sim2\times 10^{10}M_\odot$ within a cutoff radius of
$<4$ kpc, and an unusually large $M/L>50(M/L)_\odot$ to make it undetectable in
the deep Hubble Frontiers Fields image, at or close to the cluster redshift;
such a lensing galaxy, however, perturbs the adjacent lensed images in an
undesirable way.
