The most common and most likely answer to this crossword clue is the 3 letter word YAH
Y
A
H
Buddy explains!
The straight part
We know that "up yours!" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case up yours! = yah.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 YAH mean?
YAH adverb- Yes.
- An upper-class person, especially a Sloane Ranger.