  We present the characterisation of the massive cluster ClG-J$104803.7+313843$
at $z=0.76$ performed using a serendipitous XMM-Newton observation. High
redshift and massive objects represent an ideal laboratory to benchmark our
understanding of how cluster form and assembly formation driven mainly by
gravity.Leveraging the high throughput of XMM-Newton we were firstly able to
determine the redshift of the object, shedding light on ambiguous photometric
redshift associations. We investigated the morphology of this cluster which
shows signs of merging activities in the outskirts and a flat core. We also
measured the radial density profile up to $R_{500}$. With these quantities in
hand, we were able to determine the mass, $M_{500}=5.64^{+0.79}_{-0.62} \times
10^{14}M_{\odot}$, using the YX proxy. This quantity improves previous
measurement of the mass of this object by a factor of $\sim 3.5$. The
characterisation of one cluster at such mass and redshift regime is fundamental
as these objects are intrinsically rare, the number of objects discovered so
far being less than $\sim 25$. Our study highlights the importance of using
X-ray observations in combination with ancillary multi-wavelength data to
improve our understanding of high-z and massive clusters
