Opposing flat before junction (5,8)
I believe the answer is:
level crossing
'junction' is the definition.
(I know that level crossing is a type of carrefour)
'opposing flat before' is the wordplay.
'opposing' becomes 'crossing' (cross can mean to oppose or go against).
'flat' becomes 'level' (synonyms).
'before' is a charade indicator (letters next to each other).
'crossing' put after 'level' is 'LEVEL-CROSSING'.
(Other definitions for level crossing that I've seen before include "Where a road traverses a railway" , "Road-rail intersection" , "Point of intersection of a road and a railway" , "Here motorist comes by train" .)