The 2% in 2% milk (3)
I believe the answer is:
fat
'the 2% in 2% milk' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for fat that I've seen before include "Naturally oily substance" , "Portly" , "Greasy substance" , "eg Lard" , "Obese, having spare tyre etc" .)