  We present spatially resolved Atacama Large Millimeter/submillimeter Array
(ALMA) [CII] observations of the z=4.7555 submillimetre galaxy, ALESS 73.1. Our
0.5" FWHM map resolves the [CII] emitting gas which is centred close to the
active galactic nucleus (AGN). The gas kinematics are dominated by rotation but
with high turbulence, v_rot/sigma_int~3.1, and a Toomre Q parameter <1
throughout the disk. By fitting three independent thin rotating disk models to
our data, we derive a total dynamical mass of 3+-2x10^10 M_sol. This is close
to the molecular gas mass derived from previous CO(2-1) observations, and
implies a CO to H_2 conversion factor alpha_CO<2.3M_sol(K km/s/pc^2)^-1. The
mass budget also constrains the stellar mass to <3.1x10^10 M_sol, and entails a
gas fraction of f_gas>~0.4. The diameter of the dust continuum emission is <2
kpc, while the star-formation rate is as high as 1000 M_sol/yr. Combined with
our stellar mass constraint, this implies an extreme specific star formation
rate >80 Gyr^{-1}, especially since there are no clear indications of recent
merger activity. Finally, our high signal-to-noise [CII] measurement revises
the observed [NII]/[CII] ratio, which suggests a close to solar metallicity,
unless the [CII] flux contains significant contributions from HII regions. Our
observations suggest that ALESS73.1 is a nascent galaxy undergoing its first
major burst of star formation, embedded within an unstable but metal-rich gas
disk.
