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

The straight part
We know that "overshadowed" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case overshadowed = eclipsed.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 ECLIPSED mean?
ECLIPSED verb- Of astronomical bodies, to cause an eclipse.
- To overshadow; to be better or more noticeable than.
- (Irish grammar) To undergo eclipsis.