The most common and most likely answer to this crossword clue is the 8 letter word SPIFFING
I've seen this crossword clue in Independent
S
P
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F
F
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G
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 = spiffing.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 SPIFFING mean?
SPIFFING verb- (usually with up or out) To make spiffy (attractive, polished, or up-to-date)
- To reward (a salesperson) with a spiff or bonus.
- To attach a spiff or bonus to the selling of (a product)
- To throw; to heave.
- Very good, excellent.
- Smart or appealing in dress or appearance.