Keto, for one (4)
I believe the answer is:
diet
'keto for one' is the definition.
(I know that ketogenic diet is a type of high-fat diet)
This is the entire clue.
(Other definitions for diet that I've seen before include "A disciplined food regime" , "International conference" , "Legislative assembly; food regimen" , "Range of food eaten" , "Food and drink regularly consumed" .)