The most common and most likely answer to this crossword clue is the 9 letter word FUNDAMENT
F
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N
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A
M
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More clues leading to the result FUNDAMENT
- Stern woman in diverting books
- Mine, I agree, finally hit bottom
- Enjoyment lady has with holy books – sitting on this?
- Good-time girl not short of love for Fanny
- Basis for National Trust supporting party girl, perhaps
- Entertaining lady with extremely neat backside
- Entertaining theatre featuring drag queen as Bottom
- Cardinal scratching a large behind
Buddy explains!

The straight part
We know that "behind" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case behind = fundament.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 FUNDAMENT mean?
FUNDAMENT noun- Foundation.
- The bottom; the buttocks or anus.
- The underlying basis or principle for a theoretical or mathematical system.