Harass, trouble (6)
I believe the answer is:
plague
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">harass</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I know that harass can be written as plague)</span><br/ ><br/ >'<span class="explanation-format__original">trouble</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I know that plague is a type of calamity)</span><br/ ><br/ ></span>
(Other definitions for plague that I've seen before include "Outbreak of a deadly disease, or infestation of pest like locusts" , "Large-scale illness" , "Contagious disease that spreads rapidly and kills many" , "Badger" , "fatal outbreak" .)