The most common and most likely answer to this crossword clue is the 8 letter word LENIENCY
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More clues leading to the result LENIENCY
Buddy explains!

The straight part
We know that "Mercy" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mercy = leniency.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 LENIENCY mean?
LENIENCY noun- The quality of mercy or forgiveness, especially in the assignment of punishment as in a court case.
- An act of being lenient.