Belgian city banishing an idiot (5)
I believe the answer is:
twerp
'idiot' is the definition.
(both can mean a fool)
'belgian city banishing an' is the wordplay.
'belgian city' becomes 'antwerp' (Antwerp is a city in Belgium).
'banishing' indicates named letters should be taken away.
'antwerp' with 'an' removed is 'TWERP'.
(Other definitions for twerp that I've seen before include "dope" , "Ninny" , "Foolish person (slang)" , "In slang, a stupid or objectionable person" , "Twit" .)