The most common and most likely answer to this crossword clue is the 14 letter word OVERESTIMATION
I've seen this crossword clue in The Sunday Telegraph
O
V
E
R
E
S
T
I
M
A
T
I
O
N
More clues leading to the result OVERESTIMATION
Buddy explains!
The straight part
We know that "Error of judgment that’s highly inaccurate" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case error of judgment that’s highly inaccurate = overestimation.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 OVERESTIMATION mean?
OVERESTIMATION noun- An excessive estimation.