Long-running quarrel (4)
I believe the answer is:
feud
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">long-running quarrel</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 feud that I've seen before include "Long-running dispute" , "Long-running quarrel" , "Private war" , "State of prolonged mutual hostility" , "Prolonged fight between families, very Sicilian" .)