  The hard X-ray source 2PBC J0658.0-1746 was proposed as an eclipsing magnetic
cataclysmic variable of the polar type, based on optical follow-ups. We present
the first spectral and timing analysis at X-ray energies with XMM-Newton,
complemented with archival X-ray, optical, IR photometry and spectroscopy. The
X-ray emission shows bright and faint phases and total eclipses recurring every
2.38 h, consistent with optical properties. This firmly identifies 2PBC
J0658.0-1746 as an eclipsing polar, the second hard X-ray selected in the
orbital period gap. The X-ray orbital modulation changes from cycle-to-cycle
and the X-ray flux is strongly variable over the years, implying a
non-stationary mass accretion rate both on short and long timescales. The X-ray
eclipses allow to refine the orbital ephemeris with period 0.09913398(4) d, and
to constrain the binary inclination $79^{\rm\,o}\lesssim i \lesssim
90^{\rm\,o}$ and the mass ratio 0.18$\rm <M_2/M_{WD}<$0.40. A companion mass
M$_{2}=0.2-0.25\rm\,M_{\odot}$ with a radius R$_{2}=0.24-0.26\rm\,R_{\odot}$
and spectral type $\sim$M4, at D$=209^{+3}_{-2}\rm\,pc$, is derived. A lower
limit to the white dwarf mass of $\sim0.6\,\rm\,M_{\odot}$ is obtained from the
X-ray spectrum. An upper limit to the magnetic colatitude, $\beta \lesssim
50^{\rm\,o}$, and a shift in azimuth, $\psi\sim14^{\rm\,o}$, of the main
accreting pole are also estimated. The optical/IR spectral energy distribution
shows large excess in the mid-IR due to lower harmonics of cyclotron emission.
A high-state mass accretion rate
$\rm\,\sim0.4-1\times10^{-10}\,M_{\odot}\,yr^{-1}$, lower than that of
cataclysmic variables above the gap and close to that of systems below it, is
estimated. With 2PBC J0658.0-1746, the number of hard X-ray selected polars
increases to thirteen members, suggesting that they are not as rare as
previously believed.
