Treats ill (5)
I believe the answer is:
doses
'treats' is the definition.
(dosing is a kind of treating)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for doses that I've seen before include "Administers medicine to (someone)" , "Prescribed quantities of medicine" , "draughts" , "Medicates" , "Specific quantities" .)