The most common and most likely answer to this crossword clue is the 21 letter word SCOBREALSCROBE/BOREAL
S
C
O
B
R
E
A
L
S
C
R
O
B
E
/
B
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A
L
Buddy explains!
The straight part
We know that "Groove/of northerly" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case groove/of northerly = scobrealscrobe/boreal.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.