  We present the X-ray analysis of a deep ~200 ksec Chandra observation of the
compact steep spectrum radio-loud quasar 3C 186 (z=1.06) and investigate the
contribution of the unresolved radio jet to the total X-ray emission. The
spectral analysis is not conclusive on the origin of the bulk of the X-ray
emission. In order to examine the jet contribution to the X-ray flux, we model
the quasar spectral energy distribution (SED), adopting several scenarios for
the jet emission. For the values of the main physical parameters favored by the
observables, a dominant role of the jet emission in the X-ray band is ruled out
when a single zone (leptonic) scenario is adopted, even including the
contribution of the external photon fields as seed photons for inverse Compton
emission. We then consider a structured jet, with the blazar component that-
although not directly visible in the X-ray band - provides an intense field of
seed synchrotron photons Compton-scattered by electrons in a mildly
relativistic knot. In this case the whole X-ray emission can be accounted for
if we assume a blazar luminosity within the range observed from flat spectrum
radio quasars. The X-ray radiative efficiency of such (structured) jet is
intimately related to the presence of a complex velocity structure. The jet
emission can provide a significant contribution in X-rays if it decelerates
within the host galaxy, on kiloparsec scales. We discuss the implications of
this model in terms of jet dynamics and interaction with the ambient medium.
