The decay-time-dependent $CP$ asymmetry in $B0\to D\ast \pm D\mp$ decays is measured using a data set corresponding to an integrated luminosity of $9$fb$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ recorded by the LHCb detector in proton-proton collisions at centre-of-mass energies of 7, 8 and 13 TeV. The $CP$ parameters are measured as $ S_{D^*D} = -0.861 \pm 0.077,\text{(stat)} \pm 0.019 ,\text{(syst)},, $ $ \Delta S_{D^*D} = 0.019 \pm 0.075 ,\text{(stat)} \pm 0.012 ,\text{(syst)} ,, $ $ C_{D^*D} = -0.059 \pm 0.092 ,\text{(stat)} \pm 0.020 ,\text{(syst)} ,, $ $ \Delta C_{D^*D} = -0.031 \pm 0.092 ,\text{(stat)} \pm 0.016 ,\text{(syst)} ,, $ $ {\cal A}_{D^*D} = 0.008 \pm 0.014 ,\text{(stat)} \pm 0.006 ,\text{(syst)},. $ The analysis provides the most precise single measurement of $CP$ violation in this decay channel to date. All parameters are consistent with their current world average values.