Semi-mobile rabble (3)

I believe the answer is:

mob

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'rabble' is the definition.
(I know that rabble can be written as mob)

I can't explain the remainder of the clue.

(Other definitions for mob that I've seen before include "An undisciplined gang of people" , "Unruly crowd" , "Large, disorderly crowd" , "Mafiosi" , "Crime syndicate or disorderly crowd" .)

I've seen this clue in the Evening Standard.

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