  We study the mass distribution of galaxy clusters in Milgromian dynamics, or
modified Newtonian dynamics (MOND). We focus on five galaxy clusters from the
X-COP sample, for which high-quality data are available on both the baryonic
mass distribution (gas and stars) and internal dynamics (from the hydrostatic
equilibrium of hot gas and the Sunyaev-Zeldovich effect). We confirm that
galaxy clusters require additional `missing matter' in MOND, although the
required amount is drastically reduced with respect to the non-baryonic dark
matter in the context of Newtonian dynamics. We studied the spatial
distribution of the missing matter by fitting the acceleration profiles of the
clusters with a Bayesian method, finding that a physical density profile with
an inner core and an outer $r^{-4}$ decline (giving a finite total mass)
provide good fits within $\sim$1 Mpc. At larger radii, the fit results are less
satisfactory but the combination of the MOND external field effect and
hydrostatic bias (quantified as 10$\%$-40$\%$) can play a key role. The missing
mass must be more centrally concentrated than the intracluster medium (ICM).
For relaxed clusters (A1795, A2029, A2142), the ratio of missing-to-visible
mass is around $1-5$ at $R\simeq200-300$ kpc and decreases to $0.4-1.1$ at
$R\simeq2-3$ Mpc, showing that the total amount of missing mass is smaller than
or comparable to the ICM mass. For clusters with known merger signatures (A644
and A2319), this global ratio increases up to $\sim$5 but may indicate
out-of-equilibrium dynamics rather than actual missing mass. We discuss various
possibilities regarding the nature of the extra mass, in particular `missing
baryons' in the form of pressure-confined cold gas clouds with masses of
$<10^5$ M$_\odot$ and sizes of $< 50$ pc.
