Give up regular foods permanently (3,4)
I believe the answer is:
for good
'permanently' is the definition.
(synonyms)
'give up regular foods' is the wordplay.
'give up' becomes 'forgo' (to forgo something is to give it up).
'regular' means one should take alternating letters.
The alternating letters of 'foods' are 'od'.
'forgo'+'od'='FOR GOOD'
(Other definitions for for good that I've seen before include "Permanently, always" , "on the side of the angels" , "Purpose of charitable work" , "Permanently - that's not bad" , "Informally, once and for all" .)