The most common and most likely answer to this crossword clue is the 9 letter word DIETETICS
I've seen this crossword clue in The Guardian
D
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More clues leading to the result DIETETICS
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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.
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What does DIETETICS mean?
DIETETICS noun- The study of diet and nutrition in relation to health and disease.