Extremism (8)
I believe the answer is:
zealotry
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">extremism</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for zealotry that I've seen before include "Excessive devotion" , "Fanaticism" , "Fanatical behaviour" , "Extremism" , "Keenness" .)