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