Epidemic disease with high mortality (6)
I believe the answer is:
plague
'epidemic disease with high mortality' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for plague that I've seen before include "large scale affliction" , "Pester with deadly disease" , "Pestilence, scourge" , "Pestilential epidemic" , "Contagious fever" .)