  The relative importance of primordial molecular cloud fragmentation versus
large-scale accretion still remains to be assessed in the context of massive
core/star formation. Studying the kinematics of the dense gas surrounding
massive-star progenitors can tell us the extent to which large-scale flow of
material impacts the growth in mass of star-forming cores. Here we present a
comprehensive dataset of the 5500(+/-800) Msun infrared dark cloud
SDC335.579-0.272 (hereafter SDC335) which exhibits a network of cold, dense,
parsec-long filaments. Atacama Large Millimeter Array (ALMA) Cycle 0
observations reveal two massive star-forming cores, MM1 and MM2, sitting at the
centre of SDC335 where the filaments intersect. With a gas mass of
545(+770,-385) Msun contained within a source diameter of 0.05pc, MM1 is one of
the most massive, compact protostellar cores ever observed in the Galaxy. As a
whole, SDC335 could potentially form an OB cluster similar to the Trapezium
cluster in Orion. ALMA and Mopra single-dish observations of the SDC335 dense
gas furthermore reveal that the kinematics of this hub-filament system are
consistent with a global collapse of the cloud. These molecular-line data point
towards an infall velocity V_{inf} =0.7(+/-0.2) km/s, and a total mass infall
rate \dot{M}_{inf} = 2.5(+/-1.0) x 10^{-3} Msun/yr towards the central pc-size
region of SDC335. This infall rate brings 750(+/-300) Msun of gas to the centre
of the cloud per free-fall time (t_{ff}=3x10^5 yr). This is enough to double
the mass already present in the central pc-size region in 3.5(+2.2,-1.0) x
t_{ff}. These values suggest that the global collapse of SDC335 over the past
million year resulted in the formation of an early O-type star progenitor at
the centre of the cloud's gravitational potential well.
