The most common and most likely answer to this crossword clue is the 3 plus 3 plus 4 letter word ALL THE BEST
I've seen this crossword clue in Independent
A
L
L
T
H
E
B
E
S
T
More clues leading to the result ALL THE BEST
- Cheers a military officer seizing large city of old
- Tablets help cut to be put right — good luck!
- Good luck, lock's opening ancient city secured by key
- Good luck for an elite?
- Everybody’s first cheers?
- Farewell Greatest Hits album?
- Tops must be put on everyone? Good luck with that!
- A goodwill message that bells broadcast around start of Easter
- Entire ancient city has time for good health
- Good wishes given by every expert?
Buddy explains!
The straight part
We know that "Good luck with that!" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case good luck with that! = all the best.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.