The most common and most likely answer to this crossword clue is the 2 letter word TA
I've seen this crossword clue in The Daily Telegraph
T
A
More clues leading to the result TA
- A rodent from tail to head is so long
- Cheers at receiving two rises
- I'm going towards being doubled up, after retirement
- Thanks and thanks again so cheerio
- Volunteers doubling up for so long
- All the best volunteers will show up twice
- A little high-class – voila!
- Deposit losing all rights of 14
- Thank you, thank you, and farewell
- Thanks repeated and so long
Buddy explains!

The straight part
We know that "so long" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case so long = ta.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 TA mean?
TA interjection- Thanks.
- (baby talk) give (imperative)
- Indicating destination: In the direction of, and arriving at.
- Used to indicate the target or recipient of an action.
- Used to indicate result of action.
- Used to indicate a resulting feeling or emotion.
- Used after an adjective to indicate its application.
- Denotes the end of a range.
- As a.
- Used to indicate a ratio or comparison.
- Used to indicate that the preceding term is to be raised to the power of the following value; indicates exponentiation.
- (time) Preceding.
- Used to describe what something consists of or contains.
- At.
- In solfège, the lowered seventh note of a major scale (the note B-flat in the fixed-do system): te.