Alight next to a posh carriage (6)
I believe the answer is:
landau
'carriage' is the definition.
(type of horse-drawn carriage)
'alight next to a posh' is the wordplay.
'alight' becomes 'land' (I've seen this in another clue).
'next to' means one lot of letters go next to another.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'land'+'a'+'u'='LANDAU'
(Other definitions for landau that I've seen before include "Horse-drawn carriage or car with folding top" , "Old four-wheeled covered carriage" , "Four-wheeled vehicle with two folding hoods" , "Four-wheeled horse-drawn carriage" , "Transport" .)