I’m cutting green bananas for diet (7)
I believe the answer is:
regimen
'for diet' is the definition.
(a diet is a certain eating regimen)
'i'm cutting green bananas' is the wordplay.
'cutting' indicates putting letters inside (some letters cut their way into a word).
'bananas' indicates anagramming the letters (informal term for mad).
'green' anagrammed gives 'regen'.
'im' placed into 'regen' is 'REGIMEN'.
(Other definitions for regimen that I've seen before include "Course of diet, therapy" , "Systematic course of treatment, diet, etc" , "Plan for e.g. diet" , "Therapeutic programme (of diet and exercise)" , "Mere gin for a course of treatment" .)