Little house on a southeastern motorway (4)
I believe the answer is:
semi
'little house' is the definition.
(short for semi-detached)
'on a southeastern motorway' is the wordplay.
'on' means one lot of letters go next to another.
'a southeastern' becomes 'SE'.
'motorway' becomes 'mi' (resembles M1, English motorway).
'se' put next to 'mi' is 'SEMI'.
(Other definitions for semi that I've seen before include "Articulated lorry (US)" , "Prefix meaning half (4)" , "The next to final round" , "Not quite the biggest tie" , "Type of house (UK) - type of truck (US)" .)