Cause of obesity is mostly inevitable destiny (3)
I believe the answer is:
fat
'cause of obesity' is the definition.
'mostly inevitable destiny' is the wordplay.
'mostly' means to remove the last letter.
'inevitable destiny' becomes 'fate' (I've seen this before).
'fate' with its final letter taken away is 'FAT'.
'is' acts as a link.
(Other definitions for fat that I've seen before include "As Bunter" , "Overweight, stout" , "and 24: Overpaid City type" , "Cooking grease" , "Corpulent; little (chance)" .)