  In this paper we present weak lensing observations of the fields around 8 z~1
luminous radio sources. These data are searched for the lensing signatures of
clusters that are either physically associated with the radio objects, or are
foreground systems projected along the line of sight. The radio sources were
all imaged with WFPC-2/HST providing high quality shape information on large
numbers of faint galaxies around them. Statistical analysis of the coherent
shear field visible in the shapes of the faint galaxies indicates that we have
detected a weak lensing signal close to one of the targets, 3C336 at z=0.927,
with a high level of confidence. A second, independent WFPC-2 observation of
this target reinforces this detection. Our results support the earlier
suggestion of weak lensing in this field by Fort et al (1996) using
ground-based data. We also combined the shear distributions in the remaining 7
field to improve our sensitivity to weak shear signals from any structure
typically associated with these sources. We find no detectable signal and
estimate an upper limit on the maximum shear allowed by our observations. Using
an N(z) estimated from lensing analyses we convert our observed lensing signal
and limits into estimates of the masses of the various structures. We suggest
that further lensing observations of distant radio sources and their host
environments may allow the cluster L_X-mass relationship to be mapped at
high-z. This is crucial for interpreting the results of the next generation of
deep X-ray surveys, and thus constraining the redshift evolution of the cluster
mass function out to z=1.
