Next to Belgium, Europe's number one team (6)
I believe the answer is:
beside
'next to' is the definition.
(I've seen this before)
'belgium europe's number one team' is the wordplay.
'belgium' becomes 'B' (international vehicle registration code).
'europe's number one' becomes 'e' (1st letter of 'europe').
'team' becomes 'side' (both can mean a sports team).
'b'+'e'+'side'='BESIDE'
(Other definitions for beside that I've seen before include "Next to - compared with" , "- - the point" , "Very near to" , "Adjacent to" , "Next door to" .)