Treated ill (5)
I believe the answer is:
dosed
This clue might be a double definition.
'treated' is the first definition.
(similar in meaning)
'ill' is the second definition.
Both the answer and definition are adjectives. Maybe you can see a link between them that I don't see?
Can you help me to learn more?
(Other definitions for dosed that I've seen before include "Given shot" , "Administered a portion of medicine to" , "Given medicine" , "Medicated" , "Treated with medicine" .)