The most common and most likely answer to this crossword clue is the 5 plus 7 plus 5 letter word SIMON RUSSELL BEALE
I've seen this crossword clue in The Guardian
S
I
M
O
N
R
U
S
S
E
L
L
B
E
A
L
E
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 = simon russell beale.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.