  We present the first {\it XMM-Newton} observation of the classical supergiant
high-mass X-ray binary XTE J1855$-$026 taken entirely during the eclipse of the
neutron star (NS), covering the orbital phases $\phi= 0.00-0.11$. The analysis
of the data allows us to a) compare with the parameters obtained during the
existing pre eclipse observation and b) explore the back illuminated stellar
wind of the B0I type donor. The black body component, used to describe the soft
excess during pre eclipse, is not observed during eclipse. It must be then
produced near the NS or along the donor-NS line. The $0.3-10$ keV luminosity
during eclipse ($\sim 10^{34}$ erg s$^{-1}$) is 70 times lower than pre
eclipse. The intensity of the Fe K$\alpha$ line, in the average eclipse
spectrum, is $\sim 7.4$ times lower than the one measured during pre eclipse.
Since K$\alpha$ photons can not be resonantly scattered in the wind, the vast
majority of Fe K$\alpha$ emission must come from distances within $1R_{*}$ from
the NS. The eclipse spectrum is successfully modelled through the addition of
two photoionized plasmas, one with low ionization ($\log\xi_{\rm 1,cold}=0.36$)
and high emission measure ($EM_{\rm 1,cold}\approx 3\times 10^{59}$ cm$^{-3}$)
and another with high ionization ($\log\xi_{\rm 2,hot}=3.7$) and low emission
measure ($EM_{\rm hot}\approx 2\times 10^{56}$ cm$^{-3}$). Assuming that the
cold and hot gas phases are the clumps and the interclump medium of the stellar
wind, respectively, and a clump volume filling factor of $\approx [0.04-0.05]$,
typical for massive stars, a density contrast between clumps and the interclump
medium of $n_{\rm c}/n_{\rm i}\approx 180$ is deduced, in agreement with
theoretical expectations and optical-UV observations of massive star winds.
