Only half of the motorway goes to the southeast (4)
I believe the answer is:
semi
'only half' is the definition.
(I've seen this in another clue)
'the motorway goes to the southeast' is the wordplay.
'the motorway' becomes 'MI' (resembles M1, English motorway).
'goes to' is a charade indicator (letters next to each other).
'the southeast' becomes 'SE' (synonyms).
'mi' after 'se' is 'SEMI'.
'of' acts as a link.
(Other definitions for semi that I've seen before include "Prefix meaning half or partly" , "Common type of house" , "Half (pfx)" , "The next to final round" , "Articulated lorry (US)" .)