  We theoretically study a single-electron spin-valley qubit in an
electrostatically defined quantum dot in a transition metal dichalcogenide
monolayer, focusing on the example of MoS$_2$. Coupling of the qubit basis
states for coherent control is challenging, as it requires a simultaneous flip
of spin and valley. Here, we show that a tilted magnetic field together with a
short-range impurity, such as a vacancy, a substitutional defect, or an adatom,
can give rise to a coupling between the qubit basis states. This mechanism
renders the in-plane $g$-factor nonzero, and allows to control the qubit with
an in-plane ac electric field, akin to electrically driven spin resonance. We
evaluate the dependence of the in-plane $g$-factor and the electrically induced
qubit Rabi frequency on the type and position of the impurity. We reveal highly
unconventional features of the coupling mechanism, arising from
symmetry-forbidden intervalley scattering, in the case when the impurity is
located at a S site. Our results provide design guidelines for electrically
controllable qubits in two-dimensional semiconductors.
