  We analyse an 850-um SCUBA map of the environment of the z=2.39 radio galaxy
53W002, which has been shown to reside in an over-density of Ly-alpha detected
galaxies. We identify four luminous submillimeter (submm) sources within a 2.3'
(1.2Mpc at z=2.39) diameter area around the radio galaxy (which itself is a
weak submm source). We employ a 1.4-GHz map to accurately locate the
counterpart of one of these sources, SMMJ17142+5016, and identify this source
with a narrow-line AGN with an extended Ly-alpha halo at z=2.390 which is
member of the structure around 53W002. Hence SMMJ17142+5016 is the first
spectroscopically-confirmed, submm-selected companion to a high-redshift radio
galaxy. We discuss the OHS JHK spectrum of this galaxy and in addition present
five new constraints on its spectral energy distribution longward of 1um, using
these to estimate its bolometric luminosity as ~8x10^12 Lo, or a star formation
rate of ~1000Mo/yr if young stars provide the bulk of the luminosity. This
result provides direct support for the statistical detection of over-densities
of SCUBA galaxies around high-redshift radio galaxies and confirms theoretical
predictions that SCUBA galaxies, as the progenitors of massive ellipticals,
should be strongly clustered in the highest density regions of the distant
Universe.
