Operates vehicles leaving Belgium (4)
I believe the answer is:
uses
'operates' is the definition.
(I've seen this before)
'vehicles leaving belgium' is the wordplay.
'vehicles' becomes 'buses' (bus is a kind of vehicle).
'leaving' suggests deleting specific letters.
'belgium' becomes 'B' (international vehicle registration code).
'buses' with 'b' removed is 'USES'.
(Other definitions for uses that I've seen before include "'Purposes, functions (4)'" , "Customs" , "Purposes; consumes" , "Puts to a particular purpose" , "Operates" .)