The most common and most likely answer to this crossword clue is the 9 letter word CONCUBINE
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More clues leading to the result CONCUBINE
Buddy explains!

The straight part
We know that "mistress" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mistress = concubine.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 CONCUBINE mean?
CONCUBINE noun- A sexual partner, especially a woman, to whom one is not or cannot be married.
- A woman who lives with a man, but who is not a wife.
- A slave-girl or woman, kept for instance in a harem, who is held for sexual service.