The most common and most likely answer to this crossword clue is the 8 plus 4 letter word MOUNTAIN GOAT
I've seen this crossword clue in The Daily Telegraph
M
O
U
N
T
A
I
N
G
O
A
T
More clues leading to the result MOUNTAIN GOAT
Buddy explains!
The straight part
We know that "Highly capricious bounder" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case highly capricious bounder = mountain goat.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.