Does wrong a lot when treated (5)
I believe the answer is:
dosed
'treated' is the definition.
(dosing is a kind of treating)
'does wrong a lot' is the wordplay.
'wrong' indicates an anagram.
'a lot' becomes 'd'.
'does' anagrammed gives 'dose'.
'dose'+'d'='DOSED'
'when' acts as a link.
(Other definitions for dosed that I've seen before include "Administered a portion of medicine to" , "Administered a drug to" , "Given a drug" , "Treated with medicine" , "Given medicine" .)