  We present the detection of weak gravitational lensing in the field of the
radio galaxy 3C324 (z=1.206) using deep HST imaging. ~From an analysis of the
shapes of faint R=24.5-27.5 galaxies in the field we measure a weak, coherent
distortion centered close to the radio source. This shear field most likely
arises from gravitational lensing of distant field galaxies by a foreground
mass concentration. In the light of previous observations of this region, which
indicate the presence of a rich cluster around the radio source, we suggest
that the most likely candidate for the lens is the cluster associated with the
radio galaxy at z=1.2. If so, this is the most distant cluster to have been
detected by weak shear observations. Such a statement has two important
consequences. Firstly, it shows that massive, collapsed structures exist in the
high redshift Universe, and secondly that a significant fraction of the
R=24.5-27.5 field galaxy population lies beyond z=1.2.
