The 1% in 1% milk (3)
I believe the answer is:
fat
'the 1% in 1% milk' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for fat that I've seen before include "Obese, overweight" , "Adipose tissue" , "Like 10 ac" , "Naturally oily substance" , "Obese, having spare tyre etc" .)