The most common and most likely answer to this crossword clue is the 4 letter word HULL
I've seen this crossword clue in Financial Times
H
U
L
L
Buddy explains!

The straight part
We know that "northern port" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case northern port = hull.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does HULL mean?
HULL noun- The outer covering of a fruit or seed.
- Any covering.
- To remove the outer covering of a fruit or seed.
- The body or frame of a vessel, such as a ship or plane.
- (of a set A) The smallest set that possesses a particular property (such as convexity) and contains every point of A; slightly more formally, the intersection of all sets which possess the specified property and of which A is a subset.
- To drift; to be carried by the impetus of wind or water on the ship's hull alone, with sails furled.
- To hit (a ship) in the hull with cannon fire etc.