It sifts would-be film-stars, of course (6,4)
I believe the answer is:
screen test
This could be a double definition.
'it sifts would-be film-stars' is the first definition.
The definition suggests a singular noun which matches the answer.
'course' is the second definition.
The definition and answer can be both acts as well as being singular nouns.
Maybe you can see an association between them that I don't see?
'of' is the link.
Can you help me to learn more?
(Other definitions for screen test that I've seen before include "Film audition" , "Audition of prospective actor" , "in which player tries to impress" , "trial for players" , "in which player is evaluated" .)