Express contentment, parking posh luxury car (4)
I believe the answer is:
purr
'express contentment' is the definition.
(I've seen this before)
'parking posh luxury car' is the wordplay.
'parking' becomes 'p' (abbreviation on road signs).
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'luxury car' becomes 'rr' (short for Rolls Royce).
'p'+'u'+'rr'='PURR'
(Other definitions for purr that I've seen before include "Soft sound from car or cat" , "Make contented sound like cat" , "Express pleasure" , "Sound of feline content" , "Sound of a happy mog" .)