They don't play principal parts (6)

I believe the answer is:

agents

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'parts' is the definition.
The answer and definition can be both natural objects as well as being plural nouns.
Maybe you can see a link between them that I don't see?

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for agents that I've seen before include "They act for others" , "Spies" , "They operate on behalf of others" , "Factors" , "Sales reps" .)

I've seen this clue in The Irish News and The Globe and Mail.

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