The most common and most likely answer to this crossword clue is the 8 letter word MACAVITY
I've seen this crossword clue in The Times
M
A
C
A
V
I
T
Y
More clues leading to the result MACAVITY
Buddy explains!

The straight part
We know that "Mysterious cat" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mysterious cat = macavity.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.