The most common and most likely answer to this crossword clue is the 9 letter word COLLINEAR
I've seen this crossword clue in The Times
C
O
L
L
I
N
E
A
R
Buddy explains!

The straight part
We know that "In a row" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case in a row = collinear.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 COLLINEAR mean?
COLLINEAR adjective- Lying on the same straight line.
- Coaxial