Liberal leader with a posh four-wheeled carriage (6)
I believe the answer is:
landau
'four-wheeled carriage' is the definition.
(I've seen this before)
'liberal leader with a posh' is the wordplay.
'leader' suggests taking the first letters.
'with' becomes 'and' (I've seen this before**).
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-u).
The first letter of 'liberal' is 'l'.
'l'+'and'+'a'+'u'='LANDAU'
(Other definitions for landau that I've seen before include "Four-wheeled vehicle with two folding hoods" , "Four-wheeled horse-drawn carriage" , "Horse-drawn carriage or car with folding top" , "Old-fashioned four-wheeled covered carriage" , "Old four-wheeled covered carriage" .)