  We present an investigation of the dust-enshrouded activity in a sample of
X-ray selected clusters drawn from the XMM-LSS survey in the redshift range z ~
0.05 - 1.05. By taking advantage of the contiguous mid-IR coverage of the
XMM-LSS field by the Spitzer SWIRE legacy survey, we examined the distribution
and number density of mid-IR bright sources out to the cluster periphery and
its dependence on redshift to probe the obscured side of the Butcher-Oemler
effect. Toward intermediate redshift clusters we identified surprisingly high
numbers of bright 24um sources, whose photometric redshifts are compatible with
cluster membership. The stacked surface density profile of 24um sources in
clusters within four redshift bins gives evidence for an excess of bright
mid-IR sources at redshift z $\geq$ 0.4 at cluster-centric radii ~ 200 - 500
kpc. Some traces of excess appear to be present at larger radii as well.
