The most common and most likely answer to this crossword clue is the 9 letter word PALLADIUM
I've seen this crossword clue in Financial Times
P
A
L
L
A
D
I
U
M
More clues leading to the result PALLADIUM
Buddy explains!

The straight part
We know that "Rare element/London theatre" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case rare element/london theatre = palladium.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 PALLADIUM mean?
PALLADIUM noun- A safeguard.
- A chemical element (symbol Pd) with an atomic number of 46: a rare, lustrous silvery-white metal.
- A single atom of this element.