The most common and most likely answer to this crossword clue is the 3 plus 3 letter word WET ROT
I've seen this crossword clue in Financial Times
W
E
T
R
O
T
More clues leading to the result WET 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 = wet 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.