Expensive to cross Belgium for hamper! (5)
I believe the answer is:
debar
'for hamper' is the definition.
(both can mean to prevent someone doing something)
'expensive to cross belgium' is the wordplay.
'expensive' becomes 'dear' (similar in meaning).
'to cross' indicates putting letters inside (some letters cross or go round others).
'belgium' becomes 'B' (international vehicle registration code).
'dear' enclosing 'b' is 'DEBAR'.
(Other definitions for debar that I've seen before include "Exclude, prohibit" , "Prohibit (from doing something)" , "Large advert" , "Formally prevent" , "Block" .)