  The physical processes of the gamma-ray emission and particle acceleration
during the prompt phase in GRBs are still unsettled. In order to perform an
unambiguous physical modelling of observations, a clear identification of the
emission mechanism is needed. An instance of a clear identification is the
synchrotron emission during the very strong flare in GRB160821A, that occurs
during the prompt phase at 135 s. Here we show that the distribution of the
radiating electrons in this flare is initially very narrow, but later develops
a power-law tail of accelerated electrons. We thus identify for the first time
the onset of particle acceleration in a GRB jet. The flare is consistent with a
late energy release from the central engine causing an external-shock as it
encounters a preexisting ring nebula of a progenitor Wolf-Rayet star.
Relativistic forward and reverse shocks develop, leading to two distinct
emission zones with similar properties. The particle acceleration only occurs
in the forward shock, moving into the dense nebula matter. Here, the
magnetisation also decreases below the critical value, which allows for Fermi
acceleration to operate. Using this fact, we find a bulk Lorentz factor of $420
\simleq \Gamma \simleq 770$, and an emission radius of $R \sim 10^{18}$ cm,
indicating a tenuous gas of the immediate circumburst surrounding. The
observation of the onset of particle acceleration thus gives new and
independent constraints on the properties of the flow as well as on theories of
particle acceleration in collisionless astrophysical shocks.
