The most common and most likely answer to this crossword clue is the 6 plus 5 letter word DOUBLE FAULT
I've seen this crossword clue in The Daily Telegraph
D
O
U
B
L
E
F
A
U
L
T
More clues leading to the result DOUBLE FAULT
Buddy explains!

The straight part
We know that "error in court" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case error in court = double fault.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 DOUBLE FAULT mean?
DOUBLE FAULT verb- To commit a double fault.