They have their own rule, which is to be unruly! (3)

I believe the answer is:

mob

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

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

Can you help me to learn more?

(Other definitions for mob that I've seen before include "Rabble" , "Crowd round" , "Kind of cap" , "Ugly crowd" , "Horde" .)

I've seen this clue in the Evening Standard.

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