The most common and most likely answer to this crossword clue is the 13 letter word DECOMPOSITION
I've seen this crossword clue in The Times
D
E
C
O
M
P
O
S
I
T
I
O
N
Buddy explains!
The straight part
We know that "rot" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case rot = decomposition.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 DECOMPOSITION mean?
DECOMPOSITION noun- A biological process through which organic material is reduced to e.g. compost.
- The act of taking something apart, e.g. for analysis.
- The splitting (of e.g. a matrix, an atom or a compound) into constituent parts.