Fast driver, expert in Rolls Royce initially (5)
I believe the answer is:
racer
'fast driver' is the definition.
(I've seen this before)
'expert in rolls royce initially' is the wordplay.
'expert' becomes 'ace' (ace is a kind of expert**).
'in' indicates putting letters inside.
'initially' indicates taking the first letters.
The first letters of 'rolls royce' is 'rr'.
'ace' going into 'rr' is 'RACER'.
(Other definitions for racer that I've seen before include "Fast mover" , "Fast-moving animal" , "Bike, perhaps" , "Runner" , "Athlete" .)