The most common and most likely answer to this crossword clue is the 7 letter word CHEERIO
I've seen this crossword clue in The Times
C
H
E
E
R
I
O
More clues leading to the result CHEERIO
- In Italy, that eastern river is so long!
- Say this before drinking red English port
- It's said on leaving two thirds of Brie, say, and port
- Two Europeans in choir organised farewell
- Farewell from guerrilla leader to north of European port
- Last word from revolutionary with anger about nothing
- Parting word from esteemed leader in heroic novel
- Root for instance, often initially is so long
- See you offer support, with elevated call for attention
- Your good health is from good spirits and one drop of Orvieto
Buddy explains!

The straight part
We know that "See you" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case see you = cheerio.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 CHEERIO mean?
CHEERIO noun- A small saveloy often consumed with tomato sauce at parties.
- Goodbye, an interjection said upon parting.
- Hello; a greeting.