Posh car turning up in kingdom, illegally (4)
I believe the answer is:
limo
'posh car' is the definition.
(short for limousine)
'turning up in kingdom illegally' is the wordplay.
'turning up' says the letters should be written in reverse.
'in' indicates the answer is hidden within the clue.
'omil' is hidden in the letters of 'kingdom illegally'.
'omil' reversed gives 'LIMO'.
(Other definitions for limo that I've seen before include "Long motor" , "Set of rooms" , "Large luxurious car" , "Luxurious car (colloq.)" , "Stretched car, often used for partying" .)