Permanently give up excess consumption ... (3,4)
I believe the answer is:
Here is my best explanation:
'permanently' is the definition.
(I've seen this before)
'give up excess consumption' is the wordplay.
'give up' becomes 'forgo' ('forgo' can be a synonym of 'give up').
'excess consumption' becomes 'od' (abbreviation for overdose).
(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" and "on the side of the angels".)