Excellent one mile motorway in French southeast (7)
I believe the answer is:
immense
'excellent' is the definition.
(immense can mean excellent or impressive)
'one mile motorway in french southeast' is the wordplay.
'one' becomes 'I' (Roman numeral).
'mile' becomes 'm' (common abbreviation).
'motorway' becomes 'M' (as in motorway names - M1, M4 etc).
'in french' becomes 'en' ('in' in French).
'southeast' becomes 'SE' (synonyms).
'i'+'m'+'m'+'en'+'se'='IMMENSE'
(Other definitions for immense that I've seen before include "Huge, considerable" , "Jumbo" , "Very good" , "Mammoth" , "Superb" .)