  Using MIRI on-board JWST we present mid-infrared sub-arcsec imaging (MIRIM)
and spectroscopy (MRS) of the hyperluminous infrared system SPT0311-58 at
z=6.9. MIRI observations are compared with existing ALMA far-infrared continuum
and [CII]158$\mu$m imaging. Even though the ALMA observations suggests very
high star formation rates (SFR) in the eastern (E) and western (W) galaxies of
the system, the H$\alpha$ line is not detected. This, together with the
detection of the Pa$\alpha$ line, implies very high optical nebular extinction
with lower limits of 4.2 (E) and 3.9 mag (W), and even larger 5.6 (E) and 10.0
(W) for SED derived values. The extinction-corrected Pa$\alpha$ SFRs are 383
and 230M$_{\odot}$yr$^{-1}$ for the E and W galaxies, respectively. This
represents 50% of the SFRs derived from the [CII]158$\mu$m line and infrared
light for the E galaxy and as low as 6% for the W galaxy. The MIRI observations
reveal a clumpy stellar structure, with each clump having 3 to 5
$\times$10$^{9}$M$_\mathrm{\odot}$, leading to a total stellar mass of 2.0 and
1.5$\times$10$^{10}$M$_\mathrm{\odot}$ for the E and W galaxies, respectively.
The specific SFR in the stellar clumps ranges from 25 to 59Gyr$^{-1}$, which
are 3 to 10 times larger than the values measured in galaxies of similar mass
at redshifts 6 to 8. The overall gas mass fraction is
$M_\mathrm{gas}$/$M_*\sim3$, similar to that of z=4.5-6 star-forming galaxies.
The observed properties of SPT0311-58 such as the clumpy distribution at
sub(kpc) scales and the very high average extinction are similar to those
observed in low- and intermediate-z LIRGs and ULIRGs, even though SPT0311-58 is
observed only 800 Myr after the Big Bang. Massive, heavily obscured, clumpy
starburst systems like SPT0311-58 likely represent the early phases in the
formation of massive high-z bulge/spheroids and luminous quasars.
