Ostentatiously impressive wood lining old carriage (6)
I believe the answer is:
flashy
'ostentatiously impressive' is the definition.
'wood lining old carriage' is the wordplay.
'wood' becomes 'ash' (ash is a kind of wood).
'lining' indicates putting letters inside.
'old carriage' becomes 'fly' (historical type of carriage).
'ash' going inside 'fly' is 'FLASHY'.
(Other definitions for flashy that I've seen before include "Cheap and showy" , "Loud" , "Tastelessly showy" , "Vulgar" , "Cheap and ostentatious" .)