Rod’s in bar with one car designer (7)
I believe the answer is:
bugatti
'car designer' is the definition.
(Bugatti is an Italian car manufacturer)
'rod's in bar with one' is the wordplay.
'rod' becomes 'gat' (synonyms).
'in' means one lot of letters goes inside another.
'bar with' becomes 'but' (both can mean 'except for'. I am not sure about the 'with' bit.).
'one' becomes 'i' (Roman numeral).
'gat' put within 'but' is 'bugatt'.
'bugatt'+'i'='BUGATTI'
(Other definitions for bugatti that I've seen before include "Classic French car firm" , "Italian racing car pioneer" , "Vintage car" , "(Of horse) being used for breeding" , "classic racing car" .)