Gangsters' mode of operation when the bank opens (3)
I believe the answer is:
mob
'gangsters'' is the definition.
(mob can mean the Mafia)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for mob that I've seen before include "press round" , "Throng" , "Crime syndicate or disorderly crowd" , "Rabble" , "Mafiosi" .)