It's often passed to a colleague (4)

I believe the answer is:

ball

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'colleague' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for ball that I've seen before include "Party" , "20" , "Delivery" , "All-round" , "Formal dance gathering" .)

I've seen this clue in the Evening Standard.

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