Semi-mobile rabble (3)

I believe the answer is:

mob

I'm a little stuck... Click here to teach me more about this clue!

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

I don't understand the rest 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.

Want a hint initially instead of a full solution? Install my app