The most common and most likely answer to this crossword clue is the 4 plus 1 plus 2 letter word NORE - NO
I've seen this crossword clue in The Times
N
O
R
E
-
N
O
Buddy explains!
The straight part
We know that "where sailors mutinied once" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case where sailors mutinied once = nore - no.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.