The most common and most likely answer to this crossword clue is the 8 letter word NARROWLY
I've seen this crossword clue in The Guardian
N
A
R
R
O
W
L
Y
More clues leading to the result NARROWLY
Buddy explains!

The straight part
We know that "Just" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case just = narrowly.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 NARROWLY mean?
NARROWLY adverb- In a narrow manner; without flexibility or latitude.
- By a narrow margin; closely.