Fast driver expert in Rolls-Royce (5)
I believe the answer is:
racer
'fast driver' is the definition.
(I've seen this before)
'expert in rolls-royce' is the wordplay.
'expert' becomes 'ace' (ace is a kind of expert).
'in' means one lot of letters goes inside another.
'rolls royce' becomes 'rr'.
'ace' going inside 'rr' is 'RACER'.
(Other definitions for racer that I've seen before include "Speedway competitor, perhaps" , "sportsman" , "Fast competitive car or animal" , "Fast-moving snake" , "American snake" .)