  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 HST 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] emission. Using a detailed gravitational lens
model, we account for lensing amplification factor 12.+/-2.0 and find that this
intrinsically L* galaxy has a stellar mass of M*=7+/-2x10^8Mo, a dynamical mass
of Mdyn=3+/-1x10^9csc^2(i)Mo (within of 2kpc) and a star-formation rate of
42+/-8Mo/yr. 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~10^(8-9)Mo 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 30Dor. Finally, we compare
the spatially resolved nebular emission line velocity with the Ly-alpha and
UV-ISM lines and find that this galaxy is surrounded by a galactic scale
outflow in which the Ly-alpha appears redshifted by ~150km/s and the UV-ISM
lines blue-shifted by -200km/s 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.
