  We discuss a technique to strongly couple a single target quantum emitter to
a cavity mode, which is enabled by virtual excitations of a nearby mesoscopic
ensemble of emitters. A collective coupling of the latter to both the cavity
and the target emitter induces strong photon non-linearities in addition to
polariton formation, in contrast to common schemes for ensemble strong
coupling. We demonstrate that strong coupling at the level of a single emitter
can be engineered via coherent and dissipative dipolar interactions with the
ensemble, and provide realistic parameters for a possible implementation with
SiV$^{-}$ defects in diamond. Our scheme can find applications, amongst others,
in quantum information processing or in the field of cavity-assisted quantum
chemistry.
