  We study the population of star-forming clumps in A521-sys1, a $\rm z=1.04$
system gravitationally lensed by the foreground ($\rm z=0.25$) cluster Abell
0521. The galaxy presents one complete counter--image with a mean magnification
of $\rm \mu\sim4$ and a wide arc containing two partial images of A521-sys1
with magnifications reaching $\rm \mu>20$, allowing the investigations of
clumps down to scales of $\rm R_{eff}<50$ pc. We identify 18 unique clumps with
a total of 45 multiple images. Intrinsic sizes and UV magnitudes reveal clumps
with elevated surface brightnesses, comparable to similar systems at redshifts
$\rm z\gtrsim1.0$. Such clumps account for $\sim40\%$ of the galaxy UV
luminosity, implying that a significant fraction of the recent star-formation
activity is taking place there. Clump masses range from $\rm 10^6\ M_\odot$ to
$\rm 10^9\ M_\odot$ and sizes from tens to hundreds of parsec, resulting in
mass surface densities from $10$ to $\rm 10^3\ M_\odot\ pc^{-2}$, with a median
of $\rm \sim10^2\ M_\odot\ pc^{-2}$. These properties suggest that we detect
star formation taking place across a wide range of scale, from cluster
aggregates to giant star-forming complexes. We find ages of less than $100$
Myr, consistent with clumps being observed close to their natal region. The
lack of galactocentric trends with mass, mass density, or age and the lack of
old migrated clumps can be explained either by dissolution of clumps after few
$\sim100$ Myr or by stellar evolution making them fall below the detectability
limits of our data.
