The most common and most likely answer to this crossword clue is the 8 letter word CO-STARS
I've seen this crossword clue in The Guardian
C
O
-
S
T
A
R
S
More clues leading to the result CO-STARS
Buddy explains!

The straight part
We know that "Duo on screen" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case duo on screen = co-stars.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-STARS mean?
CO-STARS 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.