Holy war (7)
I believe the answer is:
crusade
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">holy war</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 crusade that I've seen before include "Drive" , "Advertising campaign" , "Action by Christians to recapture the Holy Land" , "Religious war" , "Medieval Christian military expedition to capture the Holy Land" .)