The most common and most likely answer to this crossword clue is the 6 plus 5 letter word RANDOM ERROR
I've seen this crossword clue in The Times
R
A
N
D
O
M
E
R
R
O
R
Buddy explains!
The straight part
We know that "Chance inaccuracy" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case chance inaccuracy = random error.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.