The most common and most likely answer to this crossword clue is the 9 letter word SLEEPWALK
I've seen this crossword clue in The Sunday Telegraph
S
L
E
E
P
W
A
L
K
Buddy explains!
The straight part
We know that "Act as Lady Macbeth" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case act as lady macbeth = sleepwalk.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 SLEEPWALK mean?
SLEEPWALK verb- To walk and/or perform other actions while sleeping; to somnambulate.