The most common and most likely answer to this crossword clue is the 7 letter word SECRETS
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More clues leading to the result SECRETS
Buddy explains!

The straight part
We know that "Mysteries" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mysteries = secrets.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 SECRETS mean?
SECRETS noun- A piece of knowledge that is hidden and intended to be kept hidden.
- The key or principle by which something is made clear; the knack.
- Something not understood or known.
- Private seclusion.
- (in the plural) The genital organs.
- A form of steel skullcap.
- (often in the plural) Any prayer spoken inaudibly and not aloud; especially, one of the prayers in the Mass, immediately following the "orate, fratres", said inaudibly by the celebrant.
- To make or keep secret.
- To hide secretly.