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We present the first {\it XMM-Newton} observation of the classical supergianthigh-mass X-ray binary XTE J1855$-$026 taken entirely during the eclipse of theneutron star (NS), covering the orbital phases $\phi= 0.00-0.11$. The analysisof the data allows us to a) compare with the parameters obtained during theexisting pre eclipse observation and b) explore the back illuminated stellarwind of the B0I type donor. The black body component, used to describe the softexcess during pre eclipse, is not observed during eclipse. It must be thenproduced near the NS or along the donor-NS line. The $0.3-10$ keV luminosityduring eclipse ($\sim 10^{34}$ erg s$^{-1}$) is 70 times lower than preeclipse. The intensity of the Fe K$\alpha$ line, in the average eclipsespectrum, 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 vastmajority of Fe K$\alpha$ emission must come from distances within $1R_{*}$ fromthe NS. The eclipse spectrum is successfully modelled through the addition oftwo 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 emissionmeasure ($EM_{\rm hot}\approx 2\times 10^{56}$ cm$^{-3}$). Assuming that thecold and hot gas phases are the clumps and the interclump medium of the stellarwind, respectively, and a clump volume filling factor of $\approx [0.04-0.05]$,typical for massive stars, a density contrast between clumps and the interclumpmedium of $n_{\rm c}/n_{\rm i}\approx 180$ is deduced, in agreement withtheoretical expectations and optical-UV observations of massive star winds.

The first X-ray spectrum of the HMXB XTE J1855-026 during the compact object eclipse