Left, on a liner (5)
'left on a liner' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for port that I've seen before include "Drink - left at sea" , "Grimsby, perhaps" , "Sweet dark red fortified wine taken after meal" , "Saint John, for one, > left" , "E.g. five [SWANSEA]" .)