We present a detailed study of the spatially resolved kinematics, star formation and stellar mass in a highly amplified galaxy at z = 4.92 behind the lensing cluster MS1358+62. We use the observed optical, near- and mid-infrared imaging from Hubble Space Telescope ACS & NICMOS and Spitzer IRAC to derive the stellar mass and the Gemini/NIFS IFU to investigate the velocity structure of the galaxy from the nebular [OII]λλ3726.8,3728.9 emission. Using a detailed gravitational lens model, we account for lensing amplification factor 12.5 +/- 2.0 and find that this intrinsically L* galaxy has a stellar mass of M* = 7 +/- 2 × 108Msolar, a dynamical mass of Mdyn = 3 +/- 1 × 109csc2(i)Msolar (within of 2kpc) and a star formation rate of 42 +/- 8Msolaryr-1. The source-plane UV/optical morphology of this galaxy is dominated by five discrete star-forming regions. Exploiting the dynamical information we derive masses for individual star-forming regions of Mcl ~ 108-9Msolar with sizes of ~200pc. We find that, at a fixed size, the star formation rate density within these HII regions is approximately two orders of magnitude greater than those observed in local spiral/starburst galaxies, but consistent with the most massive HII regions in the local Universe such as 30Doradus. Finally, we compare the spatially resolved nebular emission-line velocity with the Lyα and UV interstellar medium (ISM) lines and find that this galaxy is surrounded by a galactic scale outflow in which the Lyα appears redshifted by ~150kms-1 and the UV-ISM lines blueshifted by ~ -200kms-1 from the (systemic) nebular emission. The velocity structure of the outflow mirrors that of the nebular emission suggesting the outflow is young (<~15Myr), and has yet to burst out of the system. Taken together, these results suggest that this young galaxy is undergoing its first major epoch of mass assembly.