Breakaway faction (8,5)
I believe the answer is:
splinter group
'breakaway faction' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for splinter group that I've seen before include "Small organisation broken away from a larger one" , "Faction broken away from its parent organisation" , "Small breakaway faction" , "Dissenters who leave" , "Breakaway movement" .)