Semi-mobile gang (3)
I believe the answer is:
mob
'gang' is the definition.
(I know that gang can be written as mob)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for mob that I've seen before include "An undisciplined gang of people" , "Kind of cap" , "Throng" , "Crime syndicate or disorderly crowd" , "press round" .)