The most common and most likely answer to this crossword clue is the 5 plus 3 letter word QUITE MAD
I've seen this crossword clue in Financial Times
Q
U
I
T
E
M
A
D
Buddy explains!
The straight part
We know that "as Lady Macbeth" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case as lady macbeth = quite mad.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.