The most common and most likely answer to this crossword clue is the 6 letter word ROTTEN
I've seen this crossword clue in Financial Times
R
O
T
T
E
N
More clues leading to the result ROTTEN
- Mean moonshine coming up clear
- Evil is clear to men in retrospect
- Bad to rent — bad
- Terrible tenor off-key around end of concert
- Decaying rubbish starts to turn even nastier
- Hill going up given X – bad!
- Reportedly, any abstemious soldiers must get promoted in rank
- Off to get rhubarb before tucking in!
- Gone off Balls and his coveted score?
- Reverse of clean: in other words, putrid
Buddy explains!
The straight part
We know that "Decaying" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case decaying = rotten.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 ROTTEN mean?
ROTTEN adjective- Of perishable items, overridden with bacteria and other infectious agents.
- In a state of decay.
- Cruel, mean or immoral.
- Bad or terrible.
- To an extreme degree.