The most common and most likely answer to this crossword clue is the 4 letter word CNUT
C
N
U
T
More clues leading to the result CNUT
Buddy explains!
The straight part
We know that "Commonly, Norseman underestimating tides, primarily?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case commonly, norseman underestimating tides, primarily? = cnut.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.