The most common and most likely answer to this crossword clue is the 12 letter word CONFIDENTIAL
I've seen this crossword clue in The Sunday Telegraph
C
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Buddy explains!
The straight part
We know that "meant to be secret" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case meant to be secret = confidential.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 CONFIDENTIAL mean?
CONFIDENTIAL adjective- Kept, or meant to be kept, secret within a certain circle of persons; not intended to be known publicly
- Inclined to share confidences; (of things) making people inclined to share confidences; involving the sharing of confidences.
- Having someone's confidence or trust; having a position requiring trust; worthy of being trusted with confidences.