The most common and most likely answer to this crossword clue is the 3 letter word ROT
I've seen this crossword clue in The Times
R
O
T
More clues leading to the result ROT
Buddy explains!

The straight part
We know that "decay" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case decay = rot.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 ROT mean?
ROT noun- The process of becoming rotten; putrefaction.
- Decaying matter.
- Any of several diseases in which breakdown of tissue occurs.
- Verbal nonsense.
- To suffer decomposition due to biological action, especially by fungi or bacteria.
- To decline in function or utility.
- To (cause to) deteriorate in any way, as in morals; to corrupt.
- To make putrid; to cause to be wholly or partially decomposed by natural processes.
- To spend a long period of time (in an unpleasant place).
- To expose, as flax, to a process of maceration, etc., for the purpose of separating the fiber; to ret.
- To talk nonsense.