Fast driver brilliant in Rolls-Royce (5)
I believe the answer is:
racer
'fast driver' is the definition.
(I've seen this before)
'brilliant in rolls-royce' is the wordplay.
'brilliant' becomes 'ace' (ace can informally mean very good).
'in' means one lot of letters goes inside another.
'rolls royce' becomes 'rr' (short for Rolls Royce).
'ace' going into 'rr' is 'RACER'.
(Other definitions for racer that I've seen before include "Perhaps 16 [SCRAMBLER]" , "Fast mover" , "Runner" , "Fast competitive car or animal" , "Run competitor; snake" .)