The most common and most likely answer to this crossword clue is the 10 letter word CHANNELLED
I've seen this crossword clue in Financial Times
C
H
A
N
N
E
L
L
E
D
Buddy explains!
The straight part
We know that "Acted as medium" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case acted as medium = channelled.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 CHANNELLED mean?
CHANNELLED verb- To make or cut a channel or groove in.
- To direct or guide along a desired course.
- (of a spirit, as of a dead person) To serve as a medium for.
- To follow as a model, especially in a performance.