It may be lost on purpose (3)
I believe the answer is:
fat
'it may be lost on purpose' 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" , "Cooking grease" , "eg Lard" , "Like 10 ac" , "As Bunter" .)