The most common and most likely answer to this crossword clue is the 2 plus 5 letter word DU BARRY
I've seen this crossword clue in Independent
D
U
B
A
R
R
Y
Buddy explains!
The straight part
We know that "mistress of Louis XV/ (2,5)" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mistress of louis xv/ (2,5) = du barry.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.