The most common and most likely answer to this crossword clue is the 3 letter word GIT
I've seen this crossword clue in Financial Times
G
I
T
More clues leading to the result GIT
Buddy explains!

The straight part
We know that "ass" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case ass = git.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 GIT mean?
GIT noun- A silly, incompetent, stupid, annoying or childish person (usually a man).
- To get.
- To get (leave; scram).
- The channel or spout through which molten metal runs into a mould in casting.