  We present a novel method for engineering an optical clock transition that is
robust against external field fluctuations and is able to overcome limits
resulting from field inhomogeneities. The technique is based on the application
of continuous driving fields to form a pair of dressed states essentially free
of all relevant shifts. Specifically, the clock transition is robust to
magnetic shifts, quadrupole and other tensor shifts, and amplitude fluctuations
of the driving fields. The scheme is applicable to either a single ion or an
ensemble of ions, and is relevant for several types of ions, such as
$^{40}\mathrm{Ca}^{+}$, $^{88}\mathrm{Sr}^{+}$, $^{138}\mathrm{Ba}^{+}$ and
$^{176}\mathrm{Lu}^{+}$. Taking a spherically symmetric Coulomb crystal formed
by 400 $^{40}\mathrm{Ca}^{+}$ ions as an example, we show through numerical
simulations that the inhomogeneous linewidth of tens of Hertz in such a crystal
together with linear Zeeman shifts of order 10~MHz are reduced to form a
linewidth of around 1~Hz. We estimate a two-order-of-magnitude reduction in
averaging time compared to state-of-the art single ion frequency references,
assuming a probe laser fractional instability of $10^{-15}$. Furthermore, a
statistical uncertainty reaching $2.9\times 10^{-16}$ in 1~s is estimated for a
cascaded clock scheme in which the dynamically decoupled Coulomb crystal clock
stabilizes the interrogation laser for an $^{27}\mathrm{Al}^{+}$ clock.
