Belgians live here, not an idiot (5)
I believe the answer is:
twerp
'idiot' is the definition.
(I've seen this before)
'belgians live here not an' is the wordplay.
'belgians live here' becomes 'antwerp' (Belgian city).
'not' suggests deleting specific letters.
'antwerp' with 'an' taken away is 'TWERP'.
(Other definitions for twerp that I've seen before include "In slang, a stupid or objectionable person" , "Ninny" , "mug" , "Numpty" , "Foolish person (slang)" .)