Belgian city getting rid of an idiot (5)
I believe the answer is:
'idiot' is the definition.
(both can mean a fool)
'belgian city getting rid of an' is the wordplay.
'belgian city' becomes 'antwerp' (Antwerp is a city in Belgium).
'getting rid of' is a deletion indicator.
'antwerp' with 'an' taken out is 'TWERP'.
(Other definitions for twerp that I've seen before include "Silly person" , "Ninny" , "mug" , "dope" , "Pillock" .)