Motorway going over the greatest country (4)
I believe the answer is:
mali
'country' is the definition.
(Mali is an example)
'motorway going over the greatest' is the wordplay.
'motorway' becomes 'M' (as in motorway names - M1, M4 etc).
'going over' is a charade indicator (letters next to each other).
'the greatest' becomes 'ali' (Muhammad Ali was known as the Greatest).
'm'+'ali'='MALI'
(Other definitions for mali that I've seen before include "Send mail out to Africa" , "African country, former French colony" , "Land" , "part of Africa" , "W. African country" .)