The most common and most likely answer to this crossword clue is the 10 letter word OPPOSITION
I've seen this crossword clue in The Daily Telegraph
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More clues leading to the result OPPOSITION
Buddy explains!

The straight part
We know that "shadow" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case shadow = opposition.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 OPPOSITION mean?
OPPOSITION noun- The action of opposing or of being in conflict.
- An opposite or contrasting position.
- The apparent relative position of two celestial bodies when one is at an angle of 180 degrees from the other as seen from the Earth.
- A political party or movement opposed to the party or government in power.
- In United States intellectual property law, a proceeding in which an interested party seeks to prevent the registration of a trademark or patent.
- A position in which the player on the move must yield with his king allowing his opponent to advance with his own king.
- The difference of quantity or quality between two propositions having the same subject and predicate.