The most common and most likely answer to this crossword clue is the 12 letter word SPIRITUALISM
I've seen this crossword clue in Independent
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Buddy explains!
The straight part
We know that "of medium interest?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case of medium interest? = spiritualism.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 SPIRITUALISM mean?
SPIRITUALISM noun- A doctrine, opposing materialism, that claims transcendency of the divine being, the altogether spiritual character of reality and the value of inwardness of consciousness.
- A belief that the dead communicate with the living, especially through a medium. Used in a broader sense than spiritism/Kardecism.
- The quality or state of being spiritual.