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, vrot/σint ~ 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 ± 2 × 1010 M⊙. This is close to the molecular gas mass derived from previous CO(2-1) observations, and implies a CO to H2 conversion factor αCO < 2.3 M⊙ (K km s-1 pc2)-1. The mass budget also constrains the stellar mass to <3.1 × 1010 M⊙, and entails a gas fraction of fgas ≳ 0.4. The diameter of the dust continuum emission is <2 kpc, while the star-formation rate is as high as 1000 M⊙ yr-1. 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.