Expert in posh car, no slouch (5)
I believe the answer is:
racer
'no slouch' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'expert in posh car' is the wordplay.
'expert' becomes 'ace' (ace can informally mean very good).
'in' means one lot of letters goes inside another.
'posh car' becomes 'rr' (short for Rolls Royce).
'ace' inserted inside 'rr' is 'RACER'.
Can you help me to learn more?
(Other definitions for racer that I've seen before include "Participant in speed contest" , "Athlete" , "Fast-moving snake" , "American snake" , "Speedway competitor, perhaps" .)