The most common and most likely answer to this crossword clue is the 9 letter word SAFEGUARD
I've seen this crossword clue in The Guardian
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Buddy explains!

The straight part
We know that "Palladium" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case palladium = safeguard.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 SAFEGUARD mean?
SAFEGUARD noun- Something that serves as a guard or protection; a defense.
- One who, or that which, defends or protects; defence; protection.
- A safe-conduct or passport, especially in time of war.
- The monitor lizard.
- To protect, to keep safe.
- To escort safely.