Go under leaders setting up city council under Mr Big (7)
I believe the answer is:
succumb
'go under' is the definition.
(thesaurus)
'leaders setting up city council under mr big' is the wordplay.
'leaders' suggests taking the first letters.
The initial letters of 'setting up city council under mr big' is 'SUCCUMB'.
(Other definitions for succumb that I've seen before include "Give in (to temptation etc)" , "Bow down" , "Fail to resist (temptation, perhaps)" , "Yield to pressure, temptation etc" , "Be fatally overwhelmed" .)