The most common and most likely answer to this crossword clue is the 8 letter word IMPROVED
I've seen this crossword clue in The Guardian
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Buddy explains!

The straight part
We know that "better" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case better = improved.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 IMPROVED mean?
IMPROVED verb- To make (something) better; to increase the value or productivity (of something).
- To become better.
- To disprove or make void; to refute.
- To disapprove of; to find fault with; to reprove; to censure.
- To use or employ to good purpose; to turn to profitable account.
- That has been made better; enhanced.