Breakaway faction (8,5)
I believe the answer is:
splinter group
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">breakaway faction</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)