  The gravitational lens system CLASS B2108+213 has two lensed images separated
by 4.56 arcsec. Such a wide image separation suggests that the lens is either a
massive galaxy, or is composed of a group of galaxies. To investigate the
structure of the lensing potential we have carried out new high resolution
imaging of the two lensed images at 1.7 GHz with the VLBA and at 5 GHz with
global VLBI. Compact and extended emission is detected from the two lensed
images, which provides additional constraints to the lensing mass model. We
find that the data are consistent with either a single lensing galaxy, or a two
galaxy lens model that takes account of a nearby companion to the main lensing
galaxy within the Einstein radius of the system. However, for an ensemble of
global power-law mass models, those with density profiles steeper than
isothermal are a better fit. The best-fitting profile for a single spherical
mass model has a slope of $\gamma=$~2.45$_{-0.18}^{+0.19}$. The system also has
a third radio component which is coincident with the main lensing galaxy. This
component is detected at milli-arcsecond scales for the first time by the 1.7
GHz VLBA and 5 GHz global VLBI imaging. However, the third radio component is
found not to be consistent with a core lensed image because the radio spectrum
differs from the two lensed images, and its flux-density is too high when
compared to what is expected from simple mass models with a variable power-law
density profile and/or a reasonable core radius. Furthermore, 1.4 GHz imaging
of the system with the MERLIN finds extended lobe emission on either side of
the main lensing galaxy. Therefore, the radio emission from the third radio
component is almost certainly from an AGN within the main lensing galaxy, which
is classified as an FR I type radio source.
