Instrument measuring distance travelled (8)
I believe the answer is:
odometer
'instrument measuring distance travelled' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for odometer that I've seen before include "Car device that measures miles travelled" , "Measurer of distance travelled in car" , "one showing our travel?" , "Instrument for measuring distance travelled in car" , "Distance measuring device" .)