Racing driver's speed on the ground, flat out (9)
I believe the answer is:
'on the ground flat out' is the definition.
(as in lying prostrate)
'racing driver's speed' is the wordplay.
'racing driver' becomes 'prost' (racing driver Alain Prost).
'speed' becomes 'rate' (the speed at which something happens).
(Other definitions for prostrate that I've seen before include "Lying face-down" , "helpless" , "no longer able to stand" , "Flat" , "Totally exhausted" .)