  We present high-resolution (30 mas or 130 au at 4.2 kpc) Atacama Large
Millimeter/submillimeter Array observations at 1.2 mm of the disc around the
forming O-type star AFGL 4176 mm1. The disc (AFGL 4176 mm1-main) has a radius
of ~1000 au and contains significant structure, most notably a spiral arm on
its redshifted side. We fitted the observed spiral with logarithmic and
Archimedean spiral models. We find that both models can describe its structure,
but the Archimedean spiral with a varying pitch angle fits its morphology
marginally better. As well as signatures of rotation across the disc, we
observe gas arcs in CH$_3$CN that connect to other millimetre continuum sources
in the field, supporting the picture of interactions within a small cluster
around AFGL 4176 mm1-main. Using local thermodynamic equilibrium modelling of
the CH$_3$CN K-ladder, we determine the temperature and velocity field across
the disc, and thus produce a map of the Toomre stability parameter. Our results
indicate that the outer disc is gravitationally unstable and has already
fragmented or is likely to fragment in the future, possibly producing further
companions. These observations provide evidence that disc fragmentation is one
possible pathway towards explaining the high fraction of multiple systems
around high-mass stars.
