An ass in Belgium (7)
I believe the answer is:
antwerp
'in belgium' is the definition.
('Antwerp' is part of 'belgium')
'an ass' is the wordplay.
'ass' becomes 'twerp' (I've seen this before).
'an'+'twerp'='ANTWERP'
(Other definitions for antwerp that I've seen before include "Major European port" , "9/11 [OLYMPIC VENUE]" , "North Sea port" , "Rent paw out in Belgium" , "foreign city" .)