The most common and most likely answer to this crossword clue is the 10 letter word PERFECTION
I've seen this crossword clue in The Daily Telegraph
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More clues leading to the result PERFECTION
Buddy explains!
The straight part
We know that "excellence" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case excellence = perfection.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 PERFECTION mean?
PERFECTION noun- The quality or state of being perfect or complete, so that nothing substandard remains; the highest attainable state or degree of excellence
- A quality, endowment, or acquirement completely excellent; an ideal; faultlessness; especially, the divine attribute of complete excellence.
- To perfect.