The most common and most likely answer to this crossword clue is the 13 letter word PATTERN-MAKER
I've seen this crossword clue in Independent
P
A
T
T
E
R
N
-
M
A
K
E
R
Buddy explains!
The straight part
We know that "casting agent" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case casting agent = pattern-maker.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.