You'd be surprised how often the little three-letter word pops up in puzzles: rot is the crossword constructor's favorite for cluing decay. In most straightforward, quick puzzles the setter will use 'decay' to define a short, common synonym — rot, spoil, go bad — because those fit cleanly into tight grids and are instantly recognizable to solvers. I’ll often pencil in rot as my first guess when crossing letters start to line up, especially when the crossing lengths demand a compact fill. Beyond that, other surface senses get used too: a clue reading 'decay' might also expect 'wane' or 'fade' if the grid wants a sense of decline rather than biological spoilage.
If you drift into cryptic territory, 'decay' can flip roles. It can be the straight definition at either end of the clue, or it can double as an anagram indicator (e.g., something 'decayed' could be rearranged), or even a hidden indicator for letters that are rotting away inside a phrase. Sometimes setters get playful: 'Decay in the church?' could lead to 'rotunda' if wordplay supports it, or 'cavity' if it's dental. I tend to enjoy when a simple word earns multiple plausible fills across puzzles — it keeps you on your toes and reminds me that language is flexible, even for five-letter entries and crossword trivia. In short, if you see 'decay' in a puzzle, start with rot, then think broader: spoil, moulder, erode, wane — and smile at whatever mischief the setter intended.