Posh princess in cheap car (2-2-2)
I believe the answer is:
la-di-da
'posh' is the definition.
(I've seen this before)
'princess in cheap car' is the wordplay.
'princess' becomes 'di' (Princess Diana).
'in' means one lot of letters goes inside another.
'cheap car' becomes 'Lada' (Soviet Union car brand).
'di' inserted inside 'lada' is 'LA-DI-DA'.
(Other definitions for la-di-da that I've seen before include "Stuck-up" , "Pretentious, snobbish" , "Informally, having affected manner of pronunciation" , "Hoity-toity" , "I'd a lad with affected manner or pronunciation" .)