Mob pair smile treacherously and render them defenceless (10)

I believe the answer is:

immobilise

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'mob' is the definition.
Both the definition and answer are verbs in their base form.
Maybe there's an association between them I don't understand?

I can't explain the remainder of the clue.

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(Other definitions for immobilise that I've seen before include "Prevent from moving" , "put out of action" .)

I've seen this clue in The Irish Times.

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