Old car is something between Mercedes and Cortina (6)
I believe the answer is:
escort
'old car' is the definition.
'escort' can be an answer for 'car' (Ford Escort is a type of car). I am not sure about the 'old' bit.
I cannot understand how the remainder of the clue works.
(Other definitions for escort that I've seen before include "Accompany (a person)" , "Custodian" , "Companion, perhaps for protection" , "Armed guards" , "Social companion or military guard" .)