The most common and most likely answer to this crossword clue is the 9 letter word PREBIOTIC
I've seen this crossword clue in The Guardian
P
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B
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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.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does PREBIOTIC mean?
PREBIOTIC noun- (chiefly in plural) A substance in food that is hard to digest and promotes the growth of beneficial intestinal microorganisms.
- Before the advent of life.
- (of a food) Not digestible, but beneficial.