The most common and most likely answer to this crossword clue is the 8 letter word STERLING
I've seen this crossword clue in The Daily Telegraph
S
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More clues leading to the result STERLING
Buddy explains!

The straight part
We know that "Excellent" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case excellent = sterling.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 STERLING mean?
STERLING noun- The currency of the United Kingdom; especially the pound.
- Former British gold or silver coinage of a standard fineness: for gold 0.91666 and for silver 0.925.
- Sterling silver, or articles made from this material.
- A structure of pilings that protects the piers of a bridge; a starling.
- Of, or relating to British currency, or the former British coinage.
- Of, relating to, or made from sterling silver.
- Of acknowledged worth or influence; high quality; authoritative.
- Genuine; true; pure; of great value or excellence.