The most common and most likely answer to this crossword clue is the 8 plus 11 letter word BENEDICT CUMBERBATCH
I've seen this crossword clue in The Guardian
B
E
N
E
D
I
C
T
C
U
M
B
E
R
B
A
T
C
H
Buddy explains!
The straight part
We know that "Actor" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case actor = benedict cumberbatch.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.