Epidemic disease with a high death rate (6)
I believe the answer is:
plague
'epidemic disease with a high death rate' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for plague that I've seen before include "large scale affliction" , "Deadly contagious disease, or a swarm of locusts" , "Epidemic (the "Great" one being in 1665)" , "Large-scale illness" , "Pester with deadly disease" .)