Governing body that defines regime (4)
I believe the answer is:
diet
'regime' is the definition.
(eating regime)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for diet that I've seen before include "What one normally eats" , "(Perhaps, try to) reduce rump; parliament" , "Restriction of food intake" , "Try to lose weight" , "Food and drink regularly consumed" .)