Fast car, expensive car, with expert aboard (5)
I believe the answer is:
racer
'fast car' is the definition.
(race car)
'expensive car with expert aboard' is the wordplay.
'expensive car' becomes 'rr' (abbreviation for Rolls Royce).
'with expert' becomes 'ace' (ace can informally mean very good. I am not sure about the 'with' bit.).
'aboard' indicates putting letters inside.
'rr' placed around 'ace' is 'RACER'.
Can you help me to learn more?
(Other definitions for racer that I've seen before include "fast-moving vehicle" , "Greyhound" , "Runner" , "Bike, perhaps" , "Fast competitive car or animal" .)