Driving power (8)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
momentum
'driving power' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for momentum that I've seen before include "Impelling force" , "Impetus gained by a moving object" , "driving power" , "Impetus gained by movement, maybe downhill" , "Impulse" .)