Secure a posh carriage (6)
I believe the answer is:
landau
'carriage' is the definition.
(I've seen this before)
'secure a posh' is the wordplay.
'secure' becomes 'land' (I've seen this in other clues).
'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, folded-top carriage" , "A four-wheel covered carriage" , "Horse-drawn carriage or car with folding top" , "Transport" , "Four-wheeled horse-drawn carriage" .)