Crossword Genius

Where actors may be beginning (5)

Ross

I believe the answer is:

onset

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'be beginning' is the definition.
(I've seen this before)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for onset that I've seen before include "First signs" , "Beginning or early stages" , "Beginning (of disease etc)" , "Outbreak" , "origin" .)

I've seen this clue in The Guardian.
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