The most common and most likely answer to this crossword clue is the 3 plus 8 letter word BAR SINISTER
I've seen this crossword clue in The Times
B
A
R
S
I
N
I
S
T
E
R
More clues leading to the result BAR SINISTER
Buddy explains!
The straight part
We know that "a sign of shame once" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a sign of shame once = bar sinister.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.