Inelastic neutron scattering spectra of Ba1-xLaxF2+x single crystals have been measured at temperatures from 450 to 1000 degrees C. A noticeable result at x = 0.16 consists in a low-energy inelastic mode (E = 0.8 THz) which is observed at a specific wavevector only and takes the form of a quasielastic signal for other wavevectors. This is explained by the assumption that the mode comprises mobile ions and defect clusters; the characteristic wavevector corresponds to the short-distance correlation length between the clusters. For x = 0.31, this mode seems overdamped, which we explain by enhanced interactions between the clusters.