Italian river, right? Left! (4)
I believe the answer is:
port
'left' is the definition.
(left side of a ship - opposite of starboard)
'italian river right?' is the wordplay.
'italian river' becomes 'po' (river in Italy).
'right?' becomes 'rt' (abbreviation as in Rt Hon- the Right Honourable).
'po'+'rt'='PORT'
(Other definitions for port that I've seen before include "E.g. five [SWANSEA]" , "Sweet dark red fortified wine taken after meal" , "Rio, perhaps" , "Light enough to carry" , "After-dinner wine" .)