The most common and most likely answer to this crossword clue is the 9 letter word OVERCASTS
I've seen this crossword clue in The Times
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The straight part
We know that "Shadows" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case shadows = overcasts.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 OVERCASTS mean?
OVERCASTS noun- An outcast.
- A cloud covering all of the sky from horizon to horizon; cloudy.
- To overthrow.
- To cover with cloud; to overshadow; to darken.
- To make gloomy; to depress.
- To be or become cloudy.
- To transform.
- To fasten (sheets) by overcast stitching or by folding one edge over another.