With the exception of one Italian port (4)
I believe the answer is:
bari
'italian port' is the definition.
(I've seen this in another clue)
'with the exception of one' is the wordplay.
'with the exception of' becomes 'bar' (I've seen this in another clue).
'one' becomes 'i' (Roman numeral).
'bar'+'i'='BARI'
(Other definitions for bari that I've seen before include "Adriatic resort" , "Adriatic port" , "Adriatic seaport" .)