  GDS J033218.92-275302.7 (here GS-14) is a $z\sim5.5$ galaxy detected in [CII]
as part of the ALPINE survey with unusual UV spectral features that have been
interpreted as signatures of either a double stellar population or of an active
galactic nucleus (AGN). We exploited the multi-wavelength coverage of GS-14 to
investigate the properties and the origin of its emission. We performed
UV-to-NIR SED-fitting, with single/double stellar population and/or AGN
component. We analyzed the VIMOS spectrum, which shows highly-ionized emission
lines (Ovi, Nv, and Niv). The line properties have been compared with those
observed in galaxies and AGN, and with the predictions from radiation transfer
models for star-forming galaxies, AGN, and shocks. The SED-fitting provides a
total stellar mass of $M_*=(4 \pm 1) \times 10^{10} M_\odot$, an age of the
main stellar population of $\sim670 Myr$ and a recent short (8 Myr) burst of
star formation (SF) of $\sim 90 M_\odot yr^{-1}$. The Nv line has a
characteristic P-Cygni profile, which suggests a $\sim 3 Myr$ old population of
stars with a mass of $\sim 5 \times10^{7} M_\odot$. The Nv profile also shows
evidence for an additional component of nebular emission. The comparison of the
line ratios with theoretical models allows us to associate the emission with SF
or AGN, but the strong radiation field required to ionize the Ovi is more
commonly related to AGN activity. We found evidence for an old and already
evolved stellar population at $z\sim 5.5$ and show that the galaxy is
experiencing a second short burst of SF. In addition, GS-14 carries signatures
of obscured AGN activity. The AGN could be responsible for the short depletion
time of this galaxy, thus making GS-14 one of the two ALPINE sources with hints
of an active nucleus and an interesting target for future follow-ups.
