Additional cast member having little to do (5)

I believe the answer is:

extra

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'additional' is the definition.
(extra means additional or surplus)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for extra that I've seen before include "Minor actors in crowd scenes" , "Possibly a bad ball" , "someone acting in a crowd" , "Possibly overthrow" , "Studio crowd player" .)

I've seen this clue in The Times.

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