  We present the analysis of SN 2020wnt, an unusual hydrogen-poor
super-luminous supernova (SLSN-I), at a redshift of 0.032. The light curves of
SN 2020wnt are characterised by an early bump lasting $\sim5$ days, followed by
a bright main peak. The SN reaches a peak absolute magnitude of
M$_{r}^{max}=-20.52\pm0.03$ mag at $\sim77.5$ days from explosion. This
magnitude is at the lower end of the luminosity distribution of SLSNe-I, but
the rise-time is one of the longest reported to date. Unlike other SLSNe-I, the
spectra of SN 2020wnt do not show O II, but strong lines of C II and Si II are
detected. Spectroscopically, SN 2020wnt resembles the Type Ic SN 2007gr, but
its evolution is significantly slower. Comparing the bolometric light curve to
hydrodynamical models, we find that SN 2020wnt luminosity can be explained by
radioactive powering. The progenitor of SN 2020wnt is likely a massive and
extended star with a pre-SN mass of 80 M$_\odot$ and a pre-SN radius of 15
R$_\odot$ that experiences a very energetic explosion of $45\times10^{51}$ erg,
producing 4 M$_\odot$ of $^{56}$Ni. In this framework, the first peak results
from a post-shock cooling phase for an extended progenitor, and the luminous
main peak is due to a large nickel production. These characteristics are
compatible with the pair-instability SN scenario. We note, however, that a
significant contribution of interaction with circumstellar material cannot be
ruled out.
