The most common and most likely answer to this crossword clue is the 6 letter word Y-AXIS
I've seen this crossword clue in Financial Times
Y
-
A
X
I
S
Buddy explains!

The straight part
We know that "vertical" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case vertical = y-axis.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.
Crossword Answer definitions
Y-AXIS noun- The axis on a graph that is usually drawn from bottom to top and usually shows the range of values of variable dependent on one other variable, or the second of two independent variables.