Contributed to, when one joined the cast (6,1,4)

I believe the answer is:

played a part

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'joined the cast' is the definition.
The definition and answer can be both to do with fundamental existence as well as being past participle verbs.
Perhaps there's a link between them I don't understand?

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

Can you help me to learn more?

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