The most common and most likely answer to this crossword clue is the 9 letter word DIGESTION
I've seen this crossword clue in The Guardian
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Buddy explains!
The straight part
We know that "organic decomposition" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case organic decomposition = digestion.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 DIGESTION mean?
DIGESTION noun- The process, in the gastrointestinal tract, by which food is converted into substances that can be utilized by the body.
- The result of this process.
- The ability to use this process.
- The processing of decay in organic matter assisted by microorganisms.
- The assimilation and understanding of ideas.
- Generation of pus; suppuration.
- Dissolution of a sample into a solution by means of adding acid and heat.