One on a dash, nothing to prevent covering over a mile (8)
I believe the answer is:
odometer
'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).
'o'+'m'='om'
'deter' placed around 'om' is 'dometer'.
'o'+'dometer'='ODOMETER'
(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" .)