The most common and most likely answer to this crossword clue is the 12 letter word SPIRITUALIST
I've seen this crossword clue in The Guardian
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More clues leading to the result SPIRITUALIST
Buddy explains!
The straight part
We know that "Medium/Medium" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case medium/medium = spiritualist.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 SPIRITUALIST mean?
SPIRITUALIST noun- One who professes a regard for spiritual things only; one whose employment is of a spiritual character; an ecclesiastic.
- One who maintains the philosophic doctrine of spiritualism.
- (spiritism) One who practises spiritism (a.k.a. spiritualism); a believer in the possibility of communication with the dead; one who attempts to communicate with the dead.
- Pertaining to spiritualism; spiritualistic.