The most common and most likely answer to this crossword clue is the 5 plus 2 plus 6 letter word BEAST OF BURDEN
I've seen this crossword clue in The Daily Telegraph
B
E
A
S
T
O
F
B
U
R
D
E
N
More clues leading to the result BEAST OF BURDEN
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 = beast of burden.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.