The most common and most likely answer to this crossword clue is the 10 letter word CONCLUSION
I've seen this crossword clue in The Guardian
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More clues leading to the result CONCLUSION
Buddy explains!

The straight part
We know that "Last bit" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case last bit = conclusion.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 CONCLUSION mean?
CONCLUSION noun- The end, finish, close or last part of something.
- The outcome or result of a process or act.
- A decision reached after careful thought.
- In an argument or syllogism, the proposition that follows as a necessary consequence of the premises.
- An experiment, or something from which a conclusion may be drawn.
- The end or close of a pleading, e.g. the formal ending of an indictment, "against the peace", etc.
- An estoppel or bar by which a person is held to a particular position.