Crossword Genius

Be a cast member of (5)

Ross

I believe the answer is:

actin

'be a cast member of' is the definition.
(I've seen this in another clue)

This is the entire clue.

(Other definitions for actin that I've seen before include "protein" , "What can affect muscles" .)

I've seen this clue in The Wall Street Journal and the Newsday.
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