The most common and most likely answer to this crossword clue is the 12 letter word PAPIER-MACHE
I've seen this crossword clue in The Times
P
A
P
I
E
R
-
M
A
C
H
E
Buddy explains!
The straight part
We know that "artistic medium" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case artistic medium = papier-mache.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 PAPIER-MACHE mean?
PAPIER-MACHE noun- Torn-up paper and other possible materials mixed with paste or glue used to create, when the mass is dried, a durable sculptural object.