The most common and most likely answer to this crossword clue is the 12 letter word DOUBLEÂ FAULT
I've seen this crossword clue in The Guardian
D
O
U
B
L
E
Â
F
A
U
L
T
Buddy explains!

The straight part
We know that "Not a single mistake by a serviceman" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case not a single mistake by a serviceman = 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 DOUBLEÂ FAULT mean?
DOUBLEÂ FAULT verb- To commit a double fault.