splinter group 
(also splinter party)
noun
A small organization, typically a political party, that has broken away from a larger one: a splinter group of the Socialist Party...
- A splinter group, the Freedom Party, have a councillor in South Staffordshire.
- The Real IRA had already been formed by a splinter group which opposed Sinn Fein's move towards participation in government.
- Among the Sadrists is the Da'wah Iraq Organization, a splinter group that broke off from the al-Da'wah Party and which reveres the two maryred Sadrs, a representative of which will attend.