The most common and most likely answer to this crossword clue is the 9 letter word DIGESTIVE
I've seen this crossword clue in The Guardian
D
I
G
E
S
T
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V
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More clues leading to the result DIGESTIVE
Buddy explains!

The straight part
We know that "Biscuit" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case biscuit = digestive.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 DIGESTIVE mean?
DIGESTIVE noun- A substance that aids digestion.
- A digestive biscuit.
- Of, relating to, or functioning in digestion.
- That causes or promotes digestion.