US car marque not allowed new insignia (7)
I believe the answer is:
chevron
'insignia' is the definition.
(chevron is a type of symbol or insignia)
'us car marque not allowed new' is the wordplay.
'us car marque' becomes 'Chevrolet' (type of US car).
'not' is a deletion indicator.
'allowed' becomes 'let' (to let is to allow or permit).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'chevrolet' with 'let' taken away is 'chevro'.
'chevro'+'n'='CHEVRON'
(Other definitions for chevron that I've seen before include "V-shaped sleeve badge" , "Stripes" , "V-shaped pattern or device" , "American multinational energy corporation with headquarters in California" , "V-shaped insignia" .)