This morning I brewed coffee and tackled the daily puzzle, and the mercy clue jumped right out at me. My go-to pick for a straight synonym in most quick puzzles is 'pity' — it's short, common, and fits four-letter slots that editors love. If the grid shows something like I T Y or you have P T Y from crossings, 'pity' is almost certainly the fill.
That said, crosswords are sneaky. If the clue has a slightly legal or formal bent, 'clemency' or 'leniency' could be in play, and those sit in the longer slots. If the clue reads more like a plea — as in 'Have mercy!' — sometimes the puzzle wants 'spare' or even 'grace.' I usually check the crossing letters first; that alone will tip me to the right choice. For today, with no pattern given, my best bet is 'pity' — it’s the fastest fit and the one I type in first, then savor the little victory when the rest of the grid falls into place.