Sound happy turning up with a posh car (4)
I believe the answer is:
purr
'sound happy' is the definition.
(I've seen this before)
'turning up with a posh car' is the wordplay.
'turning' says the letters should be written in reverse.
'with' means one lot of letters go next to another.
'a posh car' becomes 'rr' (I've seen this in other clues).
'up' reversed gives 'pu'.
'pu'+'rr'='PURR'
(Other definitions for purr that I've seen before include "Show pleasure" , "Noise of a contented feline" , "Tiddles' sound" , "Cat's noise" , "Happy feline sound" .)