  We show that the most distant X-ray detected cluster known to date, ClJ1001
at z=2.506, hosts a strong overdensity of radio sources. Six of them are
individually detected (within 10") in deep 0.75" resolution VLA 3GHz imaging,
with S(3GHz)>8uJy. Of the six, AGN likely affects the radio emission in two
galaxies while star formation is the dominant source powering the remaining
four. We searched for cluster candidates over the full COSMOS 2-square degree
field using radio-detected 3GHz sources and looking for peaks in Sigma5 density
maps. ClJ1001 is the strongest overdensity by far with >10sigma, with a simple
z_phot>1.5 preselection. A cruder photometric rejection of z<1 radio
foregrounds leaves ClJ1001 as the second strongest overdensity, while even
using all radio sources ClJ1001 remains among the four strongest projected
overdensities. We conclude that there are great prospects for future, deep and
wide-area radio surveys to discover large samples of the first generation of
forming galaxy clusters. In these remarkable structures widespread star
formation and AGN activity of massive galaxy cluster members, residing within
the inner cluster core, will ultimately lead to radio continuum as one of the
most effective means for their identification, with detection rates expected in
the ballpark of 0.1-1 per square degree at z>2.5. Samples of hundreds such
high-redshift clusters could potentially constrain cosmological parameters and
test cluster and galaxy formation models.
