The most common and most likely answer to this crossword clue is the 16 letter word STOCKTON-ON-TEES
I've seen this crossword clue in The Times
S
T
O
C
K
T
O
N
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O
N
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T
E
E
S
More clues leading to the result STOCKTON-ON-TEES
Buddy explains!

The straight part
We know that "northern town" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case northern town = stockton-on-tees.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.