The most common and most likely answer to this crossword clue is the 12 letter word PROVIDENTIAL
I've seen this crossword clue in The Guardian
P
R
O
V
I
D
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A
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Buddy explains!
The straight part
We know that "lucky" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case lucky = providential.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 PROVIDENTIAL mean?
PROVIDENTIAL adjective- Pertaining to divine providence.
- Fortunate, as if occurring through the intervention of Providence.