Part is to act as a ball, I say (4)

I believe the answer is:

role

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'part' is the definition.
(eg a theatrical role)

I cannot understand how the remainder of the clue works.

(Other definitions for role that I've seen before include "Playing part" , "Duty" , "Stage part" , "A part of lore" , "Function; part" .)

I've seen this clue in the Sydney Morning Herald.

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