Do without overdose permanently (3,4)
I believe the answer is:
for good
'permanently' is the definition.
(I've seen this before)
'do without overdose' is the wordplay.
'do without' becomes 'forgo' (I've seen this before).
'overdose' becomes 'od' ('o.d.' can be a synonym of 'overdose').
'forgo'+'od'='FOR GOOD'
(Other definitions for for good that I've seen before include "Permanently - that's not bad" , "Purpose of charitable work" , "Permanently, always" , "Informally, once and for all" , "on the side of the angels" .)