The most common and most likely answer to this crossword clue is the 2 plus 6 letter word AU REVOIR
I've seen this crossword clue in Financial Times
A
U
R
E
V
O
I
R
More clues leading to the result AU REVOIR
- Nice farewell?
- Cheerio to Napoleon?
- Australia's opening pair, turning, run one extra bye
- Aussie opening pair, having turned, run one extra bye
- Gold ring round in ebbing water, I’m off!
- 11 of United ended setback in attitude
- See you when you arrive, but not first off
- Our Vera and I, besotted for so long
- See you in Monte Carlo
- Final words from a posh minister, one entertained by soldiers
Buddy explains!

The straight part
We know that "I’m off" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case i’m off = au revoir.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.