One on a dash, nothing to prevent covering over a mile (8)
'one on a dash' is the definition.
(device on a dashboard)
'nothing to prevent covering over a mile' is the wordplay.
'nothing' becomes 'o' (looks like zero - 0).
'to prevent' becomes 'deter' (I've seen this before).
'covering' is an insertion indicator.
'over' becomes 'o' (cricket abbreviation).
'a mile' becomes 'm' (common abbreviation).
'deter' placed around 'om' is 'dometer'.
(Other definitions for odometer that I've seen before include "Trip recorder", "Device showing a vehicle's distance travelled", "Mileometer", "Car-mileage measurer", "Travel measure".)