The most common and most likely answer to this crossword clue is the 10 letter word HANDEDNESS
I've seen this crossword clue in The Guardian
H
A
N
D
E
D
N
E
S
S
Buddy explains!

The straight part
We know that "It’s left, if not right" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it’s left, if not right = handedness.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 HANDEDNESS mean?
HANDEDNESS noun- The property that distinguishes an asymmetric object from its mirror image. For example, the essential difference between a left and right glove.
- A preference (usually innate) for using one hand rather than the other.