An idiot in Belgian port (7)
I believe the answer is:
antwerp
'belgian port' is the definition.
(I've seen this before)
'an idiot' is the wordplay.
'idiot' becomes 'twerp' (I've seen this before).
'an'+'twerp'='ANTWERP'
'in' is the link.
(Other definitions for antwerp that I've seen before include "Belgian port and province" , "European city" , "North Sea port" , "Busy Belgian port" , "harbour" .)