Terrible cot started plague (7)
I believe the answer is:
bedevil
'plague' is the definition.
(I know that bedevil is a kind of plague)
'terrible cot started' is the wordplay.
'terrible' becomes 'evil' (I am not sure about this - if you are sure you should believe this answer much more).
'cot' becomes 'bed' (cot is a type of bed).
'started' means one lot of letters go next to another.
'evil' put after 'bed' is 'BEDEVIL'.
Can you help me to learn more?
(Other definitions for bedevil that I've seen before include "Cause difficulties for, like Satan?" , "Throw into confusion" , "torment" , "Dog" , "Vile bed (anag.)" .)