Harass, trouble (6)
I believe the answer is:
plague
This clue is a double definition.
'harass' is the first definition.
(I know that harass can be written as plague)
'trouble' is the second definition.
(I know that plague is a type of calamity)
(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" .)