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

The straight part
We know that "evening" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case evening = equating.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.
Crossword Answer definitions
EQUATING verb- To consider equal or equivalent.
- To set as equal.
- The act by which things are equated; the evaluation of things as equivalent.