The most common and most likely answer to this crossword clue is the 8 letter word CHEERING
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Buddy explains!

The straight part
We know that "showing enthusiasm" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case showing enthusiasm = cheering.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 CHEERING mean?
CHEERING verb- To gladden; to make cheerful; often with up.
- To infuse life, courage, animation, or hope, into; to inspirit; to solace or comfort.
- To applaud or encourage with cheers or shouts.
- A cheer.