  We report the discovery with the Neutron Star Interior Composition Explorer
(NICER) of mHz X-ray brightness oscillations from the "clocked burster." NICER
observed the source in the periods 2017 June 20 - 29, July 11 - 13, and
September 9 - 15, for a total useful exposure of 34 ks. Two consecutive dwells
obtained on 2017 September 9 revealed highly significant oscillations at a
frequency of 8 mHz. The fractional, sinusoidal modulation amplitude increases
from 0.7 % at 1 keV to approximately 2 % at 6 keV. Similar oscillations were
also detected at lower significance in three additional dwells. The oscillation
frequency and amplitude are consistent with those of mHz QPOs reported in other
accreting neutron star systems. A thermonuclear X-ray burst was also observed
on 2017 June 22. The burst properties and X-ray colors are both consistent with
the source being in a soft spectral state during these observations, findings
that are confirmed by ongoing monitoring with MAXI and SWIFT-BAT. Assuming that
the mHz oscillations are associated with black body emission from the neutron
star surface, modeling of the phase-resolved spectra shows that the oscillation
is consistent with being produced by modulation of the temperature component of
this emission. In this interpretation, the black body normalization,
proportional to the emitting surface area, is consistent with being constant
through the oscillation cycle. We place the observations in the context of the
current theory of marginally stable burning and briefly discuss the potential
for constraining neutron star properties using mHz oscillations.
