Secure metal carriage (6)
I believe the answer is:
landau
'carriage' is the definition.
(I've seen this before)
'secure metal' is the wordplay.
'secure' becomes 'land' (I've seen this in other clues).
'metal' becomes 'au' (Au is an example**).
'land'+'au'='LANDAU'
(Other definitions for landau that I've seen before include "Four-wheeled horse-drawn carriage" , "A four-wheel covered carriage" , "Old four-wheeled covered carriage" , "An auld type of carriage" , "Transport" .)