Belgian port excluding an idiot (5)
I believe the answer is:
twerp
'idiot' is the definition.
(I've seen this before)
'belgian port excluding an' is the wordplay.
'belgian port' becomes 'antwerp' (I've seen this before).
'excluding' is a deletion indicator.
'antwerp' with 'an' removed is 'TWERP'.
(Other definitions for twerp that I've seen before include "dope" , "Fool" , "Numpty" , "Nincompoop" , "Despicable person" .)