The most common and most likely answer to this crossword clue is the 2 plus 4 letter word CO STAR
I've seen this crossword clue in Financial Times
C
O
S
T
A
R
More clues leading to the result CO STAR
Buddy explains!

The straight part
We know that "One of the top actors" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one of the top actors = co star.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 CO STAR mean?
CO STAR noun- A person who shares star billing
- A person who slightly lacks the status to be considered a star
- To perform with the billing of a costar.