The most common and most likely answer to this crossword clue is the 5 plus 1 plus 3 letter word BREAK A LEG
B
R
E
A
K
A
L
E
G
More clues leading to the result BREAK A LEG
- Best of British beer good after siesta
- Good wishes for actor resulting in 9?
- Encouragement to actor to pause on a stage
- Traditional words before play interrupt each session
- Do something that requires surgery? Good luck in theatre!
- . Good luck fixing leak in rickety barge
- Gale? Good luck at the theatre!
- d Good luck playing Akela, bizarrely, in production of 1984?
Buddy explains!

The straight part
We know that "Good luck" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case good luck = break a leg.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.