Wide thoroughfare south of Belgium (5)
I believe the answer is:
broad
'wide' is the definition.
('broad' can be a synonym of 'wide')
'thoroughfare south of belgium' is the wordplay.
'thoroughfare' becomes 'road' (I've seen this before).
'south of' means one lot of letters go next to another.
'belgium' becomes 'b' (international vehicle registration code).
'road' put after 'b' is 'BROAD'.
(Other definitions for broad that I've seen before include "Covering a wide range" , "dame" , "Spacious - tolerant" , "Board (anag.)" , "extensive" .)