Article by Guardian on metal car marque (6)
I believe the answer is:
austin
'car marque' is the definition.
(vintage car marque)
'article by guardian on metal' is the wordplay.
'article' becomes 'a' (the indefinite article in English).
'by' is a charade indicator (letters next to each other).
'guardian' becomes 'us' (the crossword setter and their Guardian colleagues).
'on' means one lot of letters go next to another.
'metal' becomes 'tin' (tin is a kind of metal).
'a'+'us'+'tin'='AUSTIN'
(Other definitions for austin that I've seen before include "Capital city of Texas" , "The capital of Texas (6)" , "US state capital - old English car manufacturer" , "State capital of Texas" , "Texan city" .)