Plague doctor in case (6)
I believe the answer is:
molest
'plague' is the definition.
('molest' can be a synonym of 'plague')
'doctor in case' is the wordplay.
'doctor' becomes 'mo' (medical officer).
'in case' becomes 'lest' (I've seen this before).
'mo'+'lest'='MOLEST'
(Other definitions for molest that I've seen before include "Attack or interfere with a person" , "Paw" , "Harry" , "Harass" , "trouble" .)